Refactor: Update event processing to not assume a single UI root

Most actions should occur in the context of the current UI root.

13 files changed, 254 insertions, 137 deletions

diff --git a/src/ui/documentwidget.c b/src/ui/documentwidget.c
index e09844c2..79678f56 100644
--- a/src/ui/documentwidget.c
+++ b/src/ui/documentwidget.c
@@ -567,7 +567,7 @@ static int scrollMax_DocumentWidget_(const iDocumentWidget *d) {
     int sm = size_GmDocument(d->doc).y - height_Rect(bounds_Widget(constAs_Widget(d))) +
              (hasSiteBanner_GmDocument(d->doc) ? 1 : 2) * d->pageMargin * gap_UI;
     if (d->phoneToolbar) {
-        sm += size_Root(get_Root()).y -
+        sm += size_Root(constAs_Widget(d)->root).y -
               top_Rect(boundsWithoutVisualOffset_Widget(d->phoneToolbar));
     }
     return sm;
@@ -1354,7 +1354,7 @@ static void scrollBegan_DocumentWidget_(iAnyObject *any, int offset, uint32_t du
     /* Show and hide toolbar on scroll. */
     if (deviceType_App() == phone_AppDeviceType) {
         if (prefs_App()->hideToolbarOnScroll && iAbs(offset) > 5) {
-            showToolbars_Root(get_Root(), offset < 0);
+            showToolbars_Root(as_Widget(d)->root, offset < 0);
         }
     }
     updateVisible_DocumentWidget_(d);
diff --git a/src/ui/inputwidget.c b/src/ui/inputwidget.c
index 52cb9805..9d360f47 100644
--- a/src/ui/inputwidget.c
+++ b/src/ui/inputwidget.c
@@ -338,7 +338,7 @@ void setText_InputWidget(iInputWidget *d, const iString *text) {
             text = enc;
         }
         /* Omit the default (Gemini) scheme if there isn't much space. */
-        if (isNarrow_Root(get_Root())) { // flags_Widget(as_Widget(d)) & tight_WidgetFlag) {
+        if (isNarrow_Root(as_Widget(d)->root)) {
             text = omitDefaultScheme_(collect_String(copy_String(text)));
         }
     }
@@ -733,7 +733,7 @@ static iBool processEvent_InputWidget_(iInputWidget *d, const SDL_Event *ev) {
         if (isFocused_Widget(d) && arg_Command(command_UserEvent(ev))) {
             iRect rect = bounds_Widget(w);
             rect.pos.y -= value_Anim(&get_Window()->rootOffset);
-            const iInt2 visRoot = visibleSize_Root(get_Root());
+            const iInt2 visRoot = visibleSize_Root(w->root);
             if (bottom_Rect(rect) > visRoot.y) {
                 setValue_Anim(&get_Window()->rootOffset, -(bottom_Rect(rect) - visRoot.y), 250);
             }
diff --git a/src/ui/lookupwidget.c b/src/ui/lookupwidget.c
index 7c3ebce2..d8e594de 100644
--- a/src/ui/lookupwidget.c
+++ b/src/ui/lookupwidget.c
@@ -427,7 +427,7 @@ void submit_LookupWidget(iLookupWidget *d, const iString *term) {
         trim_String(&d->pendingTerm);
         iReleasePtr(&d->pendingDocs);
         if (!isEmpty_String(&d->pendingTerm)) {
-            d->pendingDocs = listDocuments_App(); /* holds reference to all open tabs */
+            d->pendingDocs = listDocuments_App(get_Root()); /* holds reference to all open tabs */
             signal_Condition(&d->jobAvailable);
         }
         else {
@@ -654,7 +654,7 @@ static iBool processEvent_LookupWidget_(iLookupWidget *d, const SDL_Event *ev) {
              (equal_Command(cmd, "layout.changed") &&
               equal_Rangecc(range_Command(cmd, "id"), "navbar"))) {
         /* Position the lookup popup under the URL bar. */ {
-            const iInt2 rootSize = size_Root(get_Root());
+            const iInt2 rootSize = size_Root(w->root);
             const iRect navBarBounds = bounds_Widget(findWidget_App("navbar"));
             setFixedSize_Widget(w, init_I2(width_Widget(findWidget_App("url")),
                                            (rootSize.y - bottom_Rect(navBarBounds)) / 2));
diff --git a/src/ui/paint.c b/src/ui/paint.c
index 02914489..8843e2e2 100644
--- a/src/ui/paint.c
+++ b/src/ui/paint.c
@@ -87,8 +87,8 @@ void unsetClip_Paint(iPaint *d) {
 void drawRect_Paint(const iPaint *d, iRect rect, int color) {
     iInt2 br = bottomRight_Rect(rect);
     /* Keep the right/bottom edge visible in the window. */
-    if (br.x == d->dst->root.widget->rect.size.x) br.x--;
-    if (br.y == d->dst->root.widget->rect.size.y) br.y--;
+    if (br.x == d->dst->size.x) br.x--;
+    if (br.y == d->dst->size.y) br.y--;
     const SDL_Point edges[] = {
         { left_Rect(rect),  top_Rect(rect) },
         { br.x, top_Rect(rect) },
diff --git a/src/ui/root.c b/src/ui/root.c
index 375107c9..f91c3ba5 100644
--- a/src/ui/root.c
+++ b/src/ui/root.c
@@ -420,37 +420,37 @@ static const char *loadAnimationCStr_(void) {
     return stopSeqCStr_[loadAnimIndex_ % iElemCount(stopSeqCStr_)];
 }
 
-static uint32_t updateReloadAnimation_Window_(uint32_t interval, void *window) {
-    iUnused(window);
+static uint32_t updateReloadAnimation_Root_(uint32_t interval, void *root) {
     loadAnimIndex_++;
-    postCommand_App("window.reload.update");
+    postCommandf_App("window.reload.update root:%p", root);
     return interval;
 }
 
-static void setReloadLabel_Window_(iWindow *d, iBool animating) {
+static void setReloadLabel_Root_(iRoot *d, iBool animating) {
     const iBool isMobile = deviceType_App() != desktop_AppDeviceType;
-    iLabelWidget *label = findChild_Widget(d->root.widget, "reload");
-    updateTextCStr_LabelWidget(label, animating ? loadAnimationCStr_() :
-                                                (isMobile ? pageMenuCStr_ : reloadCStr_));
+    iLabelWidget *label = findChild_Widget(d->widget, "reload");
+    updateTextCStr_LabelWidget(
+        label, animating ? loadAnimationCStr_() : (isMobile ? pageMenuCStr_ : reloadCStr_));
     if (isMobile) {
         setCommand_LabelWidget(label,
                                collectNewCStr_String(animating ? "navigate.reload" : "menu.open"));
     }
 }
 
-static void checkLoadAnimation_Window_(iWindow *d) {
-    const iBool isOngoing = isRequestOngoing_DocumentWidget(document_App());
+static void checkLoadAnimation_Root_(iRoot *d) {
+    const iBool isOngoing = isRequestOngoing_DocumentWidget(document_Root(d));
     if (isOngoing && !d->loadAnimTimer) {
-        d->loadAnimTimer = SDL_AddTimer(loadAnimIntervalMs_, updateReloadAnimation_Window_, d);
+        d->loadAnimTimer = SDL_AddTimer(loadAnimIntervalMs_, updateReloadAnimation_Root_, d);
     }
     else if (!isOngoing && d->loadAnimTimer) {
         SDL_RemoveTimer(d->loadAnimTimer);
         d->loadAnimTimer = 0;
     }
-    setReloadLabel_Window_(d, isOngoing);
+    setReloadLabel_Root_(d, isOngoing);
 }
 
 void updatePadding_Root(iRoot *d) {
+    if (d == NULL) return;
 #if defined (iPlatformAppleMobile)
     iWidget *toolBar = findChild_Widget(d->widget, "toolbar");
     float left, top, right, bottom;
@@ -470,8 +470,6 @@ void updatePadding_Root(iRoot *d) {
            are not arranged correctly until it's hidden and reshown. */
     }
     /* Note that `handleNavBarCommands_` also adjusts padding and spacing. */
-#else
-    iUnused(d);    
 #endif
 }
 
@@ -529,7 +527,7 @@ iBool isNarrow_Root(const iRoot *d) {
 static iBool handleNavBarCommands_(iWidget *navBar, const char *cmd) {
     if (equal_Command(cmd, "window.resized") || equal_Command(cmd, "metrics.changed")) {
         const iBool isPhone = deviceType_App() == phone_AppDeviceType;
-        const iBool isNarrow = !isPhone && isNarrow_Root(get_Root());
+        const iBool isNarrow = !isPhone && isNarrow_Root(navBar->root);
         /* Adjust navbar padding. */ {
             int hPad = isPhone && isPortrait_App() ? 0 : (isPhone || isNarrow) ? gap_UI / 2
                                                                                  : gap_UI * 3 / 2;
@@ -570,7 +568,7 @@ static iBool handleNavBarCommands_(iWidget *navBar, const char *cmd) {
                     updateSize_LabelWidget(btn);
                 }
             }
-            arrange_Widget(get_Window()->root.widget);
+            arrange_Widget(navBar->root->widget);
         }
         /* Resize the URL input field. */ {
             iWidget *urlBar = findChild_Widget(navBar, "url");
@@ -582,8 +580,11 @@ static iBool handleNavBarCommands_(iWidget *navBar, const char *cmd) {
         return iFalse;
     }
     else if (equal_Command(cmd, "window.reload.update")) {
-        checkLoadAnimation_Window_(get_Window());
-        return iTrue;
+        if (pointerLabel_Command(cmd, "root") == get_Root()) {
+            checkLoadAnimation_Root_(get_Root());
+            return iTrue;
+        }
+        return iFalse;
     }
     else if (equal_Command(cmd, "navigate.focus")) {
         iWidget *url = findChild_Widget(navBar, "url");
@@ -638,7 +639,7 @@ static iBool handleNavBarCommands_(iWidget *navBar, const char *cmd) {
                 visitUrl_Visited(visited_App(), withSpacesEncoded_String(urlStr), 0); /* TODO: internal URI normalization */
                 postCommand_App("visited.changed"); /* sidebar will update */
                 setText_InputWidget(url, urlStr);
-                checkLoadAnimation_Window_(get_Window());
+                checkLoadAnimation_Root_(get_Root());
                 dismissPortraitPhoneSidebars_Root(get_Root());
                 updateNavBarIdentity_(navBar);
                 updateNavDirButtons_(navBar);
@@ -651,13 +652,13 @@ static iBool handleNavBarCommands_(iWidget *navBar, const char *cmd) {
                 return iFalse;
             }
             else if (equal_Command(cmd, "document.request.cancelled")) {
-                checkLoadAnimation_Window_(get_Window());
+                checkLoadAnimation_Root_(get_Root());
                 return iFalse;
             }
             else if (equal_Command(cmd, "document.request.started")) {
                 iInputWidget *url = findChild_Widget(navBar, "url");
                 setTextCStr_InputWidget(url, suffixPtr_Command(cmd, "url"));
-                checkLoadAnimation_Window_(get_Window());
+                checkLoadAnimation_Root_(get_Root());
                 dismissPortraitPhoneSidebars_Root(get_Root());
                 return iFalse;
             }
@@ -668,7 +669,7 @@ static iBool handleNavBarCommands_(iWidget *navBar, const char *cmd) {
         iDocumentWidget *doc = document_App();
         if (doc) {
             setText_InputWidget(findChild_Widget(navBar, "url"), url_DocumentWidget(doc));
-            checkLoadAnimation_Window_(get_Window());
+            checkLoadAnimation_Root_(get_Root());
             updateNavBarIdentity_(navBar);
         }
         setFocus_Widget(NULL);
@@ -722,7 +723,7 @@ static iBool handleSearchBarCommands_(iWidget *searchBar, const char *cmd) {
         if (pointer_Command(cmd) == findChild_Widget(searchBar, "find.input")) {
             if (!isVisible_Widget(searchBar)) {
                 setFlags_Widget(searchBar, hidden_WidgetFlag | disabled_WidgetFlag, iFalse);
-                arrange_Widget(get_Window()->root.widget);
+                arrange_Widget(root_Widget(searchBar));
                 postRefresh_App();
             }
         }
@@ -831,6 +832,9 @@ static int appIconSize_(void) {
 }
 
 void updateMetrics_Root(iRoot *d) {
+    if (d == NULL) {
+        return;
+    }
     /* Custom frame. */
     iWidget *winBar = findChild_Widget(d->widget, "winbar");
     if (winBar) {
diff --git a/src/ui/root.h b/src/ui/root.h
index 65da8d85..39e7bf3c 100644
--- a/src/ui/root.h
+++ b/src/ui/root.h
@@ -13,6 +13,7 @@ struct Impl_Root {
     iWidget *  focus;
     iPtrArray *onTop; /* order is important; last one is topmost */
     iPtrSet *  pendingDestruction;
+    int        loadAnimTimer;
 };
 
 iDeclareTypeConstruction(Root)
diff --git a/src/ui/sidebarwidget.c b/src/ui/sidebarwidget.c
index f31dd195..62d195fb 100644
--- a/src/ui/sidebarwidget.c
+++ b/src/ui/sidebarwidget.c
@@ -709,7 +709,7 @@ static void itemClicked_SidebarWidget_(iSidebarWidget *d, const iSidebarItem *it
             const iGmDocument *doc = document_DocumentWidget(document_App());
             const iGmHeading *head = constAt_Array(headings_GmDocument(doc), item->id);
             postCommandf_App("document.goto loc:%p", head->text.start);
-            dismissPortraitPhoneSidebars_Root(get_Root());
+            dismissPortraitPhoneSidebars_Root(as_Widget(d)->root);
             break;
         }
         case feeds_SidebarMode: {
@@ -792,7 +792,7 @@ void setWidth_SidebarWidget(iSidebarWidget *d, float widthAsGaps) {
         /* Even less space if the other sidebar is visible, too. */
         const int otherWidth =
             width_Widget(findWidget_App(d->side == left_SideBarSide ? "sidebar2" : "sidebar"));
-        width = iClamp(width, 30 * gap_UI, size_Root(get_Root()).x - 50 * gap_UI - otherWidth);
+        width = iClamp(width, 30 * gap_UI, size_Root(w->root).x - 50 * gap_UI - otherWidth);
     }
     d->widthAsGaps = (float) width / (float) gap_UI;
     if (isVisible_Widget(w)) {
@@ -967,7 +967,7 @@ static iBool processEvent_SidebarWidget_(iSidebarWidget *d, const SDL_Event *ev)
                     d,
                     ((d->side == left_SideBarSide
                          ? local.x
-                         : (size_Root(get_Root()).x - coord_Command(cmd).x)) +
+                          : (size_Root(w->root).x - coord_Command(cmd).x)) +
                      resMid) / (float) gap_UI);
             }
             return iTrue;
@@ -1170,7 +1170,7 @@ static iBool processEvent_SidebarWidget_(iSidebarWidget *d, const SDL_Event *ev)
         else if (isCommand_Widget(w, ev, "ident.edit")) {
             const iGmIdentity *ident = menuIdentity_SidebarWidget_(d);
             if (ident) {
-                makeValueInput_Widget(get_Window()->root.widget,
+                makeValueInput_Widget(get_Root()->widget,
                                       &ident->notes,
                                       uiHeading_ColorEscape "${heading.ident.notes}",
                                       format_CStr(cstr_Lang("dlg.ident.notes"), cstr_String(name_GmIdentity(ident))),
diff --git a/src/ui/touch.c b/src/ui/touch.c
index 6867a34b..e49fce2a 100644
--- a/src/ui/touch.c
+++ b/src/ui/touch.c
@@ -161,7 +161,7 @@ static iBool clearWidgetMomentum_TouchState_(iTouchState *d, iWidget *widget) {
 
 static void dispatchMotion_Touch_(iFloat3 pos, int buttonState) {
     touchState_()->currentTouchPos = initF3_I2(pos);
-    dispatchEvent_Widget(get_Window()->root.widget, (SDL_Event *) &(SDL_MouseMotionEvent){
+    dispatchEvent_Window(get_Window(), (SDL_Event *) &(SDL_MouseMotionEvent){
         .type = SDL_MOUSEMOTION,
         .timestamp = SDL_GetTicks(),
         .which = SDL_TOUCH_MOUSEID,
@@ -185,12 +185,12 @@ static iBool dispatchClick_Touch_(const iTouch *d, int button) {
         .x = x_F3(tapPos),
         .y = y_F3(tapPos)
     };
-    iBool wasUsed = dispatchEvent_Widget(window->root.widget, (SDL_Event *) &btn);
+    iBool wasUsed = dispatchEvent_Window(window, (SDL_Event *) &btn);
     /* Immediately released, too. */
     btn.type = SDL_MOUSEBUTTONUP;
     btn.state = SDL_RELEASED;
     btn.timestamp = SDL_GetTicks();
-    dispatchEvent_Widget(window->root.widget, (SDL_Event *) &btn);
+    dispatchEvent_Window(window, (SDL_Event *) &btn);
     if (!wasUsed && button == SDL_BUTTON_RIGHT) {
         postContextClick_Window(window, &btn);
     }
@@ -199,7 +199,7 @@ static iBool dispatchClick_Touch_(const iTouch *d, int button) {
 
 static void dispatchButtonDown_Touch_(iFloat3 pos) {
     touchState_()->currentTouchPos = initF3_I2(pos);
-    dispatchEvent_Widget(get_Window()->root.widget, (SDL_Event *) &(SDL_MouseButtonEvent){
+    dispatchEvent_Window(get_Window(), (SDL_Event *) &(SDL_MouseButtonEvent){
         .type = SDL_MOUSEBUTTONDOWN,
         .timestamp = SDL_GetTicks(),
         .clicks = 1,
@@ -213,7 +213,7 @@ static void dispatchButtonDown_Touch_(iFloat3 pos) {
 
 static void dispatchButtonUp_Touch_(iFloat3 pos) {
     touchState_()->currentTouchPos = initF3_I2(pos);
-    dispatchEvent_Widget(get_Window()->root.widget, (SDL_Event *) &(SDL_MouseButtonEvent){
+    dispatchEvent_Window(get_Window(), (SDL_Event *) &(SDL_MouseButtonEvent){
         .type = SDL_MOUSEBUTTONUP,
         .timestamp = SDL_GetTicks(),
         .clicks = 1,
@@ -227,12 +227,15 @@ static void dispatchButtonUp_Touch_(iFloat3 pos) {
 
 static void dispatchNotification_Touch_(const iTouch *d, int code) {
     if (d->affinity) {
+        iRoot *oldRoot = get_Root();
+        setCurrent_Root(d->affinity->root);
         dispatchEvent_Widget(d->affinity, (SDL_Event *) &(SDL_UserEvent){
             .type = SDL_USEREVENT,
             .timestamp = SDL_GetTicks(),
             .code = code,
             .data1 = d->affinity
         });
+        setCurrent_Root(oldRoot);
     }
 }
 
@@ -335,7 +338,7 @@ static void update_TouchState_(void *ptr) {
 }
 
 static iWidget *findOverflowScrollable_Widget_(iWidget *d) {
-    const iInt2 rootSize = size_Root(get_Root());
+    const iInt2 rootSize = size_Root(d->root);
     for (iWidget *w = d; w; w = parent_Widget(w)) {
         if (flags_Widget(w) & overflowScrollable_WidgetFlag) {
             if (height_Widget(w) > rootSize.y && !hasVisibleChildOnTop_Widget(w)) {
@@ -441,7 +444,7 @@ iBool processEvent_Touch(const SDL_Event *ev) {
     if (!isFinished_Anim(&window->rootOffset)) {
         return iFalse;
     }
-    const iInt2 rootSize = size_Root(get_Root());
+    const iInt2 rootSize = size_Window(window);
     const SDL_TouchFingerEvent *fing = &ev->tfinger;
     const iFloat3 pos = add_F3(init_F3(fing->x * rootSize.x, fing->y * rootSize.y, 0), /* pixels */
                                init_F3(0, -value_Anim(&window->rootOffset), 0));
@@ -458,7 +461,7 @@ iBool processEvent_Touch(const SDL_Event *ev) {
         else if (x > rootSize.x - edgeWidth) {
             edge = right_TouchEdge;
         }
-        iWidget *aff = hitChild_Widget(window->root.widget, init_I2(iRound(x), iRound(y_F3(pos))));
+        iWidget *aff = hitChild_Window(window, init_I2(iRound(x), iRound(y_F3(pos))));
         if (edge == left_TouchEdge) {
             dragging = findSlidePanel_Widget_(aff);
             if (dragging) {
@@ -607,7 +610,6 @@ iBool processEvent_Touch(const SDL_Event *ev) {
         }
     }
     else if (ev->type == SDL_FINGERUP) {
-        iTouch *touch = find_TouchState_(d, fing->fingerId);
         iForEach(Array, i, d->touches) {
             iTouch *touch = i.value;
             if (touch->id != fing->fingerId) {
diff --git a/src/ui/util.c b/src/ui/util.c
index 2a72d511..edfd43cc 100644
--- a/src/ui/util.c
+++ b/src/ui/util.c
@@ -635,7 +635,7 @@ static iBool menuHandler_(iWidget *menu, const char *cmd) {
         }
         if ((equal_Command(cmd, "mouse.clicked") || equal_Command(cmd, "mouse.missed")) &&
             arg_Command(cmd)) {
-            if (hitChild_Widget(get_Window()->root.widget, coord_Command(cmd)) == parentMenuButton_(menu)) {
+            if (hitChild_Window(get_Window(), coord_Command(cmd)) == parentMenuButton_(menu)) {
                 return iFalse;
             }
             /* Dismiss open menus when clicking outside them. */
@@ -1366,7 +1366,7 @@ void finalizeSheet_Widget(iWidget *sheet) {
        easier to create phone versions of each dialog, but at least this works with any
        future changes to the UI (..."works"). At least this way it is possible to enforce
        a consistent styling. */
-    if (deviceType_App() == phone_AppDeviceType && parent_Widget(sheet) == get_Window()->root.widget) {
+    if (deviceType_App() == phone_AppDeviceType && parent_Widget(sheet) == root_Widget(sheet)) {
         if (~flags_Widget(sheet) & keepOnTop_WidgetFlag) {
             /* Already finalized. */
             arrange_Widget(sheet);
@@ -1573,7 +1573,7 @@ void finalizeSheet_Widget(iWidget *sheet) {
             destroy_Widget(pageContent);
             setFlags_Widget(owner, drawBackgroundToBottom_WidgetFlag, iTrue);
         }
-        destroyPending_Root(get_Root());
+        destroyPending_Root(sheet->root);
         /* Additional elements for preferences. */
         if (isPrefs) {
             addChild_Widget(topPanel, iClob(makePadding_Widget(lineHeight_Text(defaultBig_FontId))));
@@ -1754,7 +1754,7 @@ static void acceptValueInput_(iWidget *dlg) {
 }
 
 static void updateValueInputWidth_(iWidget *dlg) {
-    const iRect safeRoot = safeRect_Root(get_Root());
+    const iRect safeRoot = safeRect_Root(dlg->root);
     const iInt2 rootSize = safeRoot.size;
     iWidget *   title    = findChild_Widget(dlg, "valueinput.title");
     iWidget *   prompt   = findChild_Widget(dlg, "valueinput.prompt");
@@ -1966,7 +1966,7 @@ iWidget *makeQuestion_Widget(const char *title, const char *msg,
     }
     addChild_Widget(dlg, iClob(makePadding_Widget(gap_UI)));
     addChild_Widget(dlg, iClob(makeDialogButtons_Widget(items, numItems)));
-    addChild_Widget(get_Window()->root.widget, iClob(dlg));
+    addChild_Widget(dlg->root->widget, iClob(dlg));
     arrange_Widget(dlg); /* BUG: This extra arrange shouldn't be needed but the dialog won't
                             be arranged correctly unless it's here. */
     finalizeSheet_Widget(dlg);
@@ -2412,7 +2412,7 @@ iWidget *makePreferences_Widget(void) {
     addChild_Widget(dlg,
                     iClob(makeDialogButtons_Widget(
                         (iMenuItem[]){ { "${dismiss}", SDLK_ESCAPE, 0, "prefs.dismiss" } }, 1)));
-    addChild_Widget(get_Window()->root.widget, iClob(dlg));
+    addChild_Widget(dlg->root->widget, iClob(dlg));
     finalizeSheet_Widget(dlg);
     //printTree_Widget(dlg);
     return dlg;
@@ -2445,7 +2445,7 @@ iWidget *makeBookmarkEditor_Widget(void) {
                                                   KMOD_PRIMARY,
                                                   "bmed.accept" } },
                                  2)));
-    addChild_Widget(get_Window()->root.widget, iClob(dlg));
+    addChild_Widget(get_Root()->widget, iClob(dlg));
     finalizeSheet_Widget(dlg);
     return dlg;
 }
@@ -2578,7 +2578,7 @@ iWidget *makeFeedSettings_Widget(uint32_t bookmarkId) {
     setId_Widget(child_Widget(buttons, childCount_Widget(buttons) - 1), "feedcfg.save");
     arrange_Widget(dlg);
     as_Widget(input)->rect.size.x = 100 * gap_UI - headings->rect.size.x;
-    addChild_Widget(get_Window()->root.widget, iClob(dlg));
+    addChild_Widget(get_Root()->widget, iClob(dlg));
     finalizeSheet_Widget(dlg);
     /* Initialize. */ {
         const iBookmark *bm  = bookmarkId ? get_Bookmarks(bookmarks_App(), bookmarkId) : NULL;
@@ -2654,7 +2654,7 @@ iWidget *makeIdentityCreation_Widget(void) {
                                          KMOD_PRIMARY,
                                          "ident.accept" } },
                         2)));
-    addChild_Widget(get_Window()->root.widget, iClob(dlg));
+    addChild_Widget(get_Root()->widget, iClob(dlg));
     finalizeSheet_Widget(dlg);
     return dlg;
 }
diff --git a/src/ui/widget.c b/src/ui/widget.c
index 2d9bf45d..5d924b57 100644
--- a/src/ui/widget.c
+++ b/src/ui/widget.c
@@ -50,6 +50,7 @@ iDefineObjectConstruction(Widget)
 
 void init_Widget(iWidget *d) {
     init_String(&d->id);
+    d->root           = get_Root(); /* never changes after this */
     d->flags          = 0;
     d->rect           = zero_Rect();
     d->minSize        = zero_I2();
@@ -81,7 +82,7 @@ void deinit_Widget(iWidget *d) {
 //#endif
     deinit_String(&d->id);
     if (d->flags & keepOnTop_WidgetFlag) {
-        removeAll_PtrArray(onTop_Root(get_Root()), d);
+        removeAll_PtrArray(onTop_Root(d->root), d);
     }
     if (d->flags & visualOffset_WidgetFlag) {
         removeTicker_App(visualOffsetAnimation_Widget_, d);
@@ -95,10 +96,10 @@ static void aboutToBeDestroyed_Widget_(iWidget *d) {
         return;
     }
     if (flags_Widget(d) & keepOnTop_WidgetFlag) {
-        removeOne_PtrArray(onTop_Root(get_Root()), d);
+        removeOne_PtrArray(onTop_Root(d->root), d);
     }
     if (isHover_Widget(d)) {
-        get_Root()->hover = NULL;
+        d->root->hover = NULL;
     }
     iForEach(ObjectList, i, d->children) {
         aboutToBeDestroyed_Widget_(as_Widget(i.object));
@@ -111,11 +112,10 @@ void destroy_Widget(iWidget *d) {
             postRefresh_App();
         }
         aboutToBeDestroyed_Widget_(d);
-        iRoot *root = get_Root();
-        if (!root->pendingDestruction) {
-            root->pendingDestruction = new_PtrSet();
+        if (!d->root->pendingDestruction) {
+            d->root->pendingDestruction = new_PtrSet();
         }
-        insert_PtrSet(root->pendingDestruction, d);
+        insert_PtrSet(d->root->pendingDestruction, d);
     }
 }
 
@@ -139,7 +139,7 @@ void setFlags_Widget(iWidget *d, int64_t flags, iBool set) {
         }
         iChangeFlags(d->flags, flags, set);
         if (flags & keepOnTop_WidgetFlag) {
-            iPtrArray *onTop = onTop_Root(get_Root());
+            iPtrArray *onTop = onTop_Root(d->root);
             if (set) {
                 iAssert(indexOf_PtrArray(onTop, d) == iInvalidPos);
                 pushBack_PtrArray(onTop, d);
@@ -182,12 +182,16 @@ void setPadding_Widget(iWidget *d, int left, int top, int right, int bottom) {
     d->padding[3] = bottom;
 }
 
+iWidget *root_Widget(const iWidget *d) {
+    return d ? d->root->widget : NULL;
+}
+
 void showCollapsed_Widget(iWidget *d, iBool show) {
     const iBool isVisible = !(d->flags & hidden_WidgetFlag);
     if ((isVisible && !show) || (!isVisible && show)) {
         setFlags_Widget(d, hidden_WidgetFlag, !show);
         /* The entire UI may be affected, if parents are resized due to the (un)collapsing. */
-        arrange_Widget(get_Window()->root.widget);
+        arrange_Widget(root_Widget(d));
         postRefresh_App();
     }
 }
@@ -352,7 +356,7 @@ static size_t numArrangedChildren_Widget_(const iWidget *d) {
 
 static void centerHorizontal_Widget_(iWidget *d) {
     d->rect.pos.x = ((d->parent ? width_Rect(innerRect_Widget_(d->parent))
-                                : size_Root(get_Root()).x) -
+                                : size_Root(d->root).x) -
                      width_Rect(d->rect)) /
                     2;
     TRACE(d, "center horizontally: %d", d->rect.pos.x);
@@ -706,7 +710,7 @@ iBool containsExpanded_Widget(const iWidget *d, iInt2 coord, int expand) {
     const iRect bounds = {
         zero_I2(),
         addY_I2(d->rect.size,
-                d->flags & drawBackgroundToBottom_WidgetFlag ? size_Root(get_Root()).y : 0)
+                d->flags & drawBackgroundToBottom_WidgetFlag ? size_Root(d->root).y : 0)
     };
     return contains_Rect(expand ? expanded_Rect(bounds, init1_I2(expand)) : bounds,
                          localCoord_Widget(d, coord));
@@ -734,24 +738,25 @@ static iBool filterEvent_Widget_(const iWidget *d, const SDL_Event *ev) {
 }
 
 void unhover_Widget(void) {
+    /* TODO: Which root? */
     get_Root()->hover = NULL;
 }
 
 iBool dispatchEvent_Widget(iWidget *d, const SDL_Event *ev) {
+    iAssert(d->root == get_Root());
     if (!d->parent) {
-        //setCurrent_Root(d);
         if (ev->type == SDL_MOUSEMOTION) {
             /* Hover widget may change. */
             setHover_Widget(NULL);
         }
-        if (get_Root()->focus && isKeyboardEvent_(ev)) {
+        if (d->root->focus && isKeyboardEvent_(ev)) {
             /* Root dispatches keyboard events directly to the focused widget. */
-            if (dispatchEvent_Widget(get_Root()->focus, ev)) {
+            if (dispatchEvent_Widget(d->root->focus, ev)) {
                 return iTrue;
             }
         }
         /* Root offers events first to widgets on top. */
-        iReverseForEach(PtrArray, i, get_Root()->onTop) {
+        iReverseForEach(PtrArray, i, d->root->onTop) {
             iWidget *widget = *i.value;
             if (isVisible_Widget(widget) && dispatchEvent_Widget(widget, ev)) {
 #if 0
@@ -775,7 +780,7 @@ iBool dispatchEvent_Widget(iWidget *d, const SDL_Event *ev) {
         }
     }
     else if (ev->type == SDL_MOUSEMOTION &&
-             (!get_Root()->hover || hasParent_Widget(d, get_Root()->hover)) &&
+             (!d->root->hover || hasParent_Widget(d, d->root->hover)) &&
              flags_Widget(d) & hover_WidgetFlag && ~flags_Widget(d) & hidden_WidgetFlag &&
              ~flags_Widget(d) & disabled_WidgetFlag) {
         if (contains_Widget(d, init_I2(ev->motion.x, ev->motion.y))) {
@@ -793,7 +798,7 @@ iBool dispatchEvent_Widget(iWidget *d, const SDL_Event *ev) {
            handle the events first. */
         iReverseForEach(ObjectList, i, d->children) {
             iWidget *child = as_Widget(i.object);
-            if (child == get_Root()->focus && isKeyboardEvent_(ev)) {
+            if (child == d->root->focus && isKeyboardEvent_(ev)) {
                 continue; /* Already dispatched. */
             }
             if (isVisible_Widget(child) && child->flags & keepOnTop_WidgetFlag) {
@@ -845,8 +850,8 @@ iBool dispatchEvent_Widget(iWidget *d, const SDL_Event *ev) {
 
 static iBool scrollOverflow_Widget_(iWidget *d, int delta) {
     iRect bounds = bounds_Widget(d);
-    const iInt2 rootSize = size_Root(get_Root());
-    const iRect winRect = safeRect_Root(get_Root());
+    const iInt2 rootSize = size_Root(d->root);
+    const iRect winRect = safeRect_Root(d->root);
     const int yTop = top_Rect(winRect);
     const int yBottom = bottom_Rect(winRect);
     //const int safeBottom = rootSize.y - yBottom;
@@ -979,14 +984,14 @@ void drawBackground_Widget(const iWidget *d) {
                 break;
         }
         fillRect_Paint(&p,
-                       rect_Root(get_Root()),
+                       rect_Root(d->root),
                        fadeColor);
         SDL_SetRenderDrawBlendMode(renderer_Window(get_Window()), SDL_BLENDMODE_NONE);
     }
     if (d->bgColor >= 0 || d->frameColor >= 0) {
         iRect rect = bounds_Widget(d);
         if (d->flags & drawBackgroundToBottom_WidgetFlag) {
-            rect.size.y = size_Root(get_Root()).y - top_Rect(rect);
+            rect.size.y = size_Root(d->root).y - top_Rect(rect);
         }
         iPaint p;
         init_Paint(&p);
@@ -994,7 +999,7 @@ void drawBackground_Widget(const iWidget *d) {
 #if defined (iPlatformAppleMobile)
             if (d->flags & (drawBackgroundToHorizontalSafeArea_WidgetFlag |
                             drawBackgroundToVerticalSafeArea_WidgetFlag)) {
-                const iInt2 rootSize = size_Root(get_Root());
+                const iInt2 rootSize = size_Root(d->root);
                 const iInt2 center = divi_I2(rootSize, 2);
                 int top = 0, right = 0, bottom = 0, left = 0;
                 if (d->flags & drawBackgroundToHorizontalSafeArea_WidgetFlag) {
@@ -1058,7 +1063,7 @@ void drawChildren_Widget(const iWidget *d) {
     }
     /* Root draws the on-top widgets on top of everything else. */
     if (!d->parent) {
-        iConstForEach(PtrArray, i, onTop_Root(get_Root())) {
+        iConstForEach(PtrArray, i, onTop_Root(d->root)) {
             const iWidget *top = *i.value;
             draw_Widget(top);
         }
@@ -1175,7 +1180,7 @@ iAny *hitChild_Widget(const iWidget *d, iInt2 coord) {
     }
     /* Check for on-top widgets first. */
     if (!d->parent) {
-        iReverseForEach(PtrArray, i, onTop_Root(get_Root())) {
+        iReverseForEach(PtrArray, i, onTop_Root(d->root)) {
             iWidget *child = i.ptr;
 //            printf("ontop: %s (%s) hidden:%d hittable:%d\n", cstr_String(id_Widget(child)),
 //                   class_Widget(child)->name,
@@ -1263,12 +1268,12 @@ iBool isDisabled_Widget(const iAnyObject *d) {
 
 iBool isFocused_Widget(const iAnyObject *d) {
     iAssert(isInstance_Object(d, &Class_Widget));
-    return get_Root()->focus == d;
+    return ((const iWidget *) d)->root->focus == d;
 }
 
 iBool isHover_Widget(const iAnyObject *d) {
     iAssert(isInstance_Object(d, &Class_Widget));
-    return get_Root()->hover == d;
+    return ((const iWidget *) d)->root->hover == d;
 }
 
 iBool isSelected_Widget(const iAnyObject *d) {
@@ -1314,13 +1319,15 @@ iBool isAffectedByVisualOffset_Widget(const iWidget *d) {
 }
 
 void setFocus_Widget(iWidget *d) {
-    if (get_Root()->focus != d) {
-        if (get_Root()->focus) {
-            iAssert(!contains_PtrSet(get_Root()->pendingDestruction, get_Root()->focus));
-            postCommand_Widget(get_Root()->focus, "focus.lost");
+    iRoot *root = d ? d->root : get_Root();
+    if (root->focus != d) {
+        if (root->focus) {
+            iAssert(!contains_PtrSet(root->pendingDestruction, root->focus));
+            postCommand_Widget(root->focus, "focus.lost");
         }
-        get_Root()->focus = d;
+        root->focus = d;
         if (d) {
+            iAssert(root == d->root);
             iAssert(flags_Widget(d) & focusable_WidgetFlag);
             postCommand_Widget(d, "focus.gained");
         }
@@ -1332,7 +1339,12 @@ iWidget *focus_Widget(void) {
 }
 
 void setHover_Widget(iWidget *d) {
-    get_Root()->hover = d;
+    if (d) {
+        d->root->hover = d;
+    }
+    else {
+        get_Root()->hover = NULL;
+    }
 }
 
 iWidget *hover_Widget(void) {
@@ -1380,7 +1392,7 @@ static const iWidget *findFocusRoot_Widget_(const iWidget *d) {
 }
 
 iAny *findFocusable_Widget(const iWidget *startFrom, enum iWidgetFocusDir focusDir) {
-    const iWidget *root = findFocusRoot_Widget_(get_Window()->root.widget);
+    const iWidget *root = findFocusRoot_Widget_(get_Root()->widget);
     iAssert(root != NULL);
     iBool getNext = (startFrom ? iFalse : iTrue);
     const iWidget *found = findFocusable_Widget_(root, startFrom, &getNext, focusDir);
@@ -1392,14 +1404,21 @@ iAny *findFocusable_Widget(const iWidget *startFrom, enum iWidgetFocusDir focusD
 }
 
 void setMouseGrab_Widget(iWidget *d) {
-    if (get_Root()->mouseGrab != d) {
-        get_Root()->mouseGrab = d;
+    iRoot *root = d ? d->root : get_Root();
+    if (root->mouseGrab != d) {
+        root->mouseGrab = d;
         SDL_CaptureMouse(d != NULL);
     }
 }
 
 iWidget *mouseGrab_Widget(void) {
-    return get_Root()->mouseGrab;
+    iWindow *win = get_Window();    
+    iForIndices(i, win->roots) {
+        if (win->roots[i] && win->roots[i]->mouseGrab) {
+            return win->roots[i]->mouseGrab;            
+        }
+    }
+    return NULL;
 }
 
 void postCommand_Widget(const iAnyObject *d, const char *cmd, ...) {
@@ -1433,7 +1452,7 @@ void refresh_Widget(const iAnyObject *d) {
 }
 
 void raise_Widget(iWidget *d) {
-    iPtrArray *onTop = onTop_Root(get_Root());
+    iPtrArray *onTop = onTop_Root(d->root);
     if (d->flags & keepOnTop_WidgetFlag) {
         iAssert(indexOf_PtrArray(onTop, d) != iInvalidPos);
         removeOne_PtrArray(onTop, d);
diff --git a/src/ui/widget.h b/src/ui/widget.h
index ab08a9b2..f126d40d 100644
--- a/src/ui/widget.h
+++ b/src/ui/widget.h
@@ -34,6 +34,8 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. */
 #include <the_Foundation/string.h>
 #include <SDL_events.h>
 
+iDeclareType(Root) /* each widget is associated with a Root */
+
 #define iDeclareWidgetClass(className) \
     iDeclareType(className); \
     typedef iWidgetClass i##className##Class; \
@@ -138,6 +140,7 @@ struct Impl_Widget {
     iObjectList *children;
     iWidget *    parent;
     iBool (*commandHandler)(iWidget *, const char *);
+    iRoot *      root;
 };
 
 iDeclareObjectConstruction(Widget)
@@ -166,7 +169,8 @@ void    destroy_Widget          (iWidget *); /* widget removed and deleted later
 void    destroyPending_Widget   (void);
 void    releaseChildren_Widget  (iWidget *);
 
-const iString *id_Widget                (const iWidget *);
+iWidget *       root_Widget             (const iWidget *);
+const iString * id_Widget               (const iWidget *);
 int64_t flags_Widget                    (const iWidget *);
 iRect   bounds_Widget                   (const iWidget *); /* outer bounds */
 iRect   innerBounds_Widget              (const iWidget *);
diff --git a/src/ui/window.c b/src/ui/window.c
index 5c10fea6..e47b579f 100644
--- a/src/ui/window.c
+++ b/src/ui/window.c
@@ -161,13 +161,40 @@ static void removeMacMenus_(void) {
 }
 #endif
 
+static int numRoots_Window(const iWindow *d) {
+    int num = 0;
+    iForIndices(i, d->roots) {
+        if (d->roots[i]) num++;
+    }
+    return num;
+}
+
 static void setupUserInterface_Window(iWindow *d) {
 #if defined (iPlatformAppleDesktop)
     insertMacMenus_();
 #endif
-    setCurrent_Root(&d->root);
-    createUserInterface_Root(&d->root);
-    setCurrent_Root(NULL);
+    iForIndices(i, d->roots) {
+        d->roots[i] = new_Root();
+        setCurrent_Root(d->roots[i]);
+        createUserInterface_Root(d->roots[i]);
+        setCurrent_Root(NULL);
+    }
+}
+
+static void windowSizeChanged_Window_(iWindow *d) {
+    const int numRoots = numRoots_Window(d);
+    /* Horizontal split frame. */
+    const iInt2 rootSize = div_I2(d->size, init_I2(numRoots, 1));
+    iForIndices(i, d->roots) {
+        iRoot *root = d->roots[i];
+        if (root) {
+            root->widget->rect.pos  = init_I2(rootSize.x * i, 0);
+            root->widget->rect.size = rootSize; /* Reposition roots. */
+            root->widget->minSize   = rootSize;
+            updatePadding_Root(root);
+            arrange_Widget(root->widget);
+        }
+    }   
 }
 
 static void updateSize_Window_(iWindow *d, iBool notifyAlways) {
@@ -175,13 +202,10 @@ static void updateSize_Window_(iWindow *d, iBool notifyAlways) {
     const iInt2 oldSize = *size;
     SDL_GetRendererOutputSize(d->render, &size->x, &size->y);
     size->y -= d->keyboardHeight;
-    d->root.widget->rect.size = *size; /* Reposition roots. */
-    d->root.widget->minSize   = *size;
     if (notifyAlways || !isEqual_I2(oldSize, *size)) {
-        updatePadding_Root(&d->root);
+        windowSizeChanged_Window_(d);
         const iBool isHoriz = (d->place.lastNotifiedSize.x != size->x);
         const iBool isVert  = (d->place.lastNotifiedSize.y != size->y);
-        arrange_Widget(d->root.widget);
         postCommandf_App("window.resized width:%d height:%d horiz:%d vert:%d",
                          size->x,
                          size->y,
@@ -196,8 +220,8 @@ static void updateSize_Window_(iWindow *d, iBool notifyAlways) {
 void drawWhileResizing_Window(iWindow *d, int w, int h) {
     /* This is called while a window resize is in progress, so we can be pretty confident
        the size has actually changed. */
-    d->root.widget->rect.size = coord_Window(d, w, h);
-    arrange_Widget(d->root.widget);
+    d->size = init_I2(w, h);
+    windowSizeChanged_Window_(d);
     draw_Window(d);
 }
 
@@ -348,7 +372,7 @@ void init_Window(iWindow *d, iRect rect) {
     theWindow_ = d;
     d->win = NULL;
     d->size = zero_I2(); /* will be updated below */
-    init_Root(&d->root);
+    iZap(d->roots);
     iZap(d->cursors);
     d->place.initialPos = rect.pos;
     d->place.normalRect = rect;
@@ -452,13 +476,17 @@ void init_Window(iWindow *d, iRect rect) {
 }
 
 void deinit_Window(iWindow *d) {
-    setCurrent_Root(&d->root);
     iRecycle();
     if (theWindow_ == d) {
         theWindow_ = NULL;
     }
-    deinit_Root(&d->root);
-    setCurrent_Root(NULL);
+    iForIndices(i, d->roots) {
+        if (d->roots[i]) {
+            setCurrent_Root(d->roots[i]);
+            deinit_Root(d->roots[i]);
+        }
+    }
+    setCurrent_Root(NULL);        
     deinit_Text();
     SDL_DestroyRenderer(d->render);
     SDL_DestroyWindow(d->win);
@@ -483,6 +511,19 @@ iBool isFullscreen_Window(const iWindow *d) {
     return snap_Window(d) == fullscreen_WindowSnap;
 }
 
+iRoot *findRoot_Window(const iWindow *d, const iWidget *widget) {
+    while (widget->parent) {
+        widget = widget->parent;
+    }
+    iForIndices(i, d->roots) {
+        if (d->roots[i] && d->roots[i]->widget == widget) {
+            return d->roots[i];
+        }
+    }
+    iAssert(iFalse); /* it must be under some Root */
+    return NULL;
+}
+
 static void invalidate_Window_(iWindow *d) {
     iUnused(d);
     resetFonts_Text();
@@ -639,7 +680,6 @@ static iBool handleWindowEvent_Window_(iWindow *d, const SDL_WindowEvent *ev) {
             return iTrue;
         }
         case SDL_WINDOWEVENT_RESIZED:
-            updatePadding_Root(&d->root);
             if (d->isMinimized) {
                 updateSize_Window_(d, iTrue);
                 return iTrue;
@@ -771,17 +811,25 @@ iBool processEvent_Window(iWindow *d, const SDL_Event *ev) {
                 event.button.x = pos.x;
                 event.button.y = pos.y;
             }
-            setCurrent_Root(&d->root);
-            iWidget *widget = d->root.widget;
+            const iWidget *oldHovers[2] = {
+                d->roots[0]->hover,
+                d->roots[1]->hover,
+            };
+            iBool wasUsed = iFalse;
+            /* Dispatch first to the mouse-grabbed widget. */
+//            iWidget *widget = d->root.widget;
             if (event.type == SDL_MOUSEMOTION || event.type == SDL_MOUSEWHEEL ||
                 event.type == SDL_MOUSEBUTTONUP || event.type == SDL_MOUSEBUTTONDOWN) {
                 if (mouseGrab_Widget()) {
-                    widget = mouseGrab_Widget();
+                    iWidget *grabbed = mouseGrab_Widget();
+                    setCurrent_Root(findRoot_Window(d, grabbed));
+                    wasUsed = dispatchEvent_Widget(grabbed, &event);
                 }
             }
-            iWidget *oldHover = hover_Widget();
             /* Dispatch the event to the tree of widgets. */
-            iBool wasUsed = dispatchEvent_Widget(widget, &event);
+            if (!wasUsed) {
+                wasUsed = dispatchEvent_Window(d, &event);
+            }
             if (!wasUsed) {
                 /* As a special case, clicking the middle mouse button can be used for pasting
                    from the clipboard. */
@@ -793,14 +841,16 @@ iBool processEvent_Window(iWindow *d, const SDL_Event *ev) {
                     paste.key.keysym.mod = KMOD_PRIMARY;
                     paste.key.state      = SDL_PRESSED;
                     paste.key.timestamp  = SDL_GetTicks();
-                    wasUsed = dispatchEvent_Widget(widget, &paste);
+                    wasUsed = dispatchEvent_Window(d, &paste);
                 }
                 if (event.type == SDL_MOUSEBUTTONDOWN && event.button.button == SDL_BUTTON_RIGHT) {
                     postContextClick_Window(d, &event.button);
                 }
             }
             if (isMetricsChange_UserEvent(&event)) {
-                updateMetrics_Root(&d->root);
+                iForIndices(i, d->roots) {                    
+                    updateMetrics_Root(d->roots[i]);
+                }
             }
             if (isCommand_UserEvent(&event, "lang.changed")) {
 #if defined (iPlatformAppleDesktop)
@@ -809,11 +859,14 @@ iBool processEvent_Window(iWindow *d, const SDL_Event *ev) {
                 insertMacMenus_();
 #endif
                 invalidate_Window_(d);
-                updatePreferencesLayout_Widget(findChild_Widget(d->root.widget, "prefs"));
-                arrange_Widget(d->root.widget);
-                //printTree_Widget(findChild_Widget(d->root, "prefs"));
+                iForIndices(i, d->roots) {
+                    if (d->roots[i]) {
+                        updatePreferencesLayout_Widget(findChild_Widget(d->roots[i]->widget, "prefs"));
+                        arrange_Widget(d->roots[i]->widget);
+                    }
+                }
             }
-            if (oldHover != hover_Widget()) {
+            if (oldHovers[0] != d->roots[0]->hover || oldHovers[1] != d->roots[1]->hover) {
                 postRefresh_App();
             }
             if (event.type == SDL_MOUSEMOTION) {
@@ -825,9 +878,35 @@ iBool processEvent_Window(iWindow *d, const SDL_Event *ev) {
     return iFalse;
 }
 
+iBool dispatchEvent_Window(iWindow *d, const SDL_Event *ev) {
+    /* TODO: Dispatch to input-focused root first. */
+    iForIndices(i, d->roots) {
+        if (d->roots[i]) {
+            setCurrent_Root(d->roots[i]);
+            const iBool wasUsed = dispatchEvent_Widget(d->roots[i]->widget, ev);
+            if (wasUsed) {
+                return iTrue;
+            }
+        }
+    }
+    return iFalse;
+}
+
+iAnyObject *hitChild_Window(const iWindow *d, iInt2 coord) {
+    iForIndices(i, d->roots) {
+        if (d->roots[i]) {
+            iAnyObject *hit = hitChild_Widget(d->roots[i]->widget, coord);
+            if (hit) {
+                return hit;
+            }
+        }
+    }
+    return NULL;
+}
+
 iBool postContextClick_Window(iWindow *d, const SDL_MouseButtonEvent *ev) {
     /* A context menu may still get triggered here. */
-    const iWidget *hit = hitChild_Widget(d->root.widget, init_I2(ev->x, ev->y));
+    const iWidget *hit = hitChild_Window(d, init_I2(ev->x, ev->y));
     while (hit && isEmpty_String(id_Widget(hit))) {
         hit = parent_Widget(hit);
     }
@@ -872,31 +951,36 @@ void draw_Window(iWindow *d) {
     /* Draw widgets. */
     d->frameTime = SDL_GetTicks();
     if (isExposed_Window(d)) {
-        setCurrent_Root(&d->root);
-        draw_Widget(d->root.widget);
+        iForIndices(i, d->roots) {
+            iRoot *root = d->roots[i];
+            if (root) {
+                setCurrent_Root(root);
+                draw_Widget(root->widget);
 #if defined (LAGRANGE_ENABLE_CUSTOM_FRAME)
-        /* App icon. */
-        const iWidget *appIcon = findChild_Widget(d->root.widget, "winbar.icon");
-        if (isVisible_Widget(appIcon)) {
-            const int   size    = appIconSize_();
-            const iRect rect    = bounds_Widget(appIcon);
-            const iInt2 mid     = mid_Rect(rect);
-            const iBool isLight = isLight_ColorTheme(colorTheme_App());
-            iColor iconColor    = get_Color(gotFocus || isLight ? white_ColorId : uiAnnotation_ColorId);
-            SDL_SetTextureColorMod(d->appIcon, iconColor.r, iconColor.g, iconColor.b);
-            SDL_SetTextureAlphaMod(d->appIcon, gotFocus || !isLight ? 255 : 92);
-            SDL_RenderCopy(
-                d->render,
-                d->appIcon,
-                NULL,
-                &(SDL_Rect){ left_Rect(rect) + gap_UI * 1.25f, mid.y - size / 2, size, size });
+                /* App icon. */
+                const iWidget *appIcon = findChild_Widget(root->widget, "winbar.icon");
+                if (isVisible_Widget(appIcon)) {
+                    const int   size    = appIconSize_();
+                    const iRect rect    = bounds_Widget(appIcon);
+                    const iInt2 mid     = mid_Rect(rect);
+                    const iBool isLight = isLight_ColorTheme(colorTheme_App());
+                    iColor iconColor    = get_Color(gotFocus || isLight ? white_ColorId : uiAnnotation_ColorId);
+                    SDL_SetTextureColorMod(d->appIcon, iconColor.r, iconColor.g, iconColor.b);
+                    SDL_SetTextureAlphaMod(d->appIcon, gotFocus || !isLight ? 255 : 92);
+                    SDL_RenderCopy(
+                        d->render,
+                        d->appIcon,
+                        NULL,
+                        &(SDL_Rect){ left_Rect(rect) + gap_UI * 1.25f, mid.y - size / 2, size, size });
+                }
+#endif                
+            }
         }
-#endif
         setCurrent_Root(NULL);
     }
 #if 0
     /* Text cache debugging. */ {
-        SDL_Rect rect = { d->root.widget->rect.size.x - 640, 0, 640, 2.5 * 640 };
+        SDL_Rect rect = { d->roots[0]->widget->rect.size.x - 640, 0, 640, 2.5 * 640 };
         SDL_SetRenderDrawColor(d->render, 0, 0, 0, 255);
         SDL_RenderFillRect(d->render, &rect);
         SDL_RenderCopy(d->render, glyphCache_Text(), NULL, &rect);
@@ -912,7 +996,7 @@ void resize_Window(iWindow *d, int w, int h) {
 
 void setTitle_Window(iWindow *d, const iString *title) {
     SDL_SetWindowTitle(d->win, cstr_String(title));
-    iLabelWidget *bar = findChild_Widget(d->root.widget, "winbar.title");
+    iLabelWidget *bar = findChild_Widget(get_Root()->widget, "winbar.title");
     if (bar) {
         updateText_LabelWidget(bar, title);
     }
diff --git a/src/ui/window.h b/src/ui/window.h
index a95d9f40..12c540d3 100644
--- a/src/ui/window.h
+++ b/src/ui/window.h
@@ -67,7 +67,7 @@ struct Impl_Window {
     uint32_t      focusGainedAt;
     SDL_Renderer *render;
     iInt2         size;
-    iRoot         root;         /* root widget and UI state */
+    iRoot *       roots[2];     /* root widget and UI state; second one is for split mode */
     float         pixelRatio;   /* conversion between points and pixels, e.g., coords, window size */
     float         displayScale; /* DPI-based scaling factor of current display, affects uiScale only */
     float         uiScale;
@@ -83,6 +83,7 @@ struct Impl_Window {
 };
 
 iBool       processEvent_Window     (iWindow *, const SDL_Event *);
+iBool       dispatchEvent_Window    (iWindow *, const SDL_Event *);
 void        draw_Window             (iWindow *);
 void        drawWhileResizing_Window(iWindow *d, int w, int h); /* workaround for SDL bug */
 void        resize_Window           (iWindow *, int w, int h);
@@ -101,10 +102,12 @@ iInt2       maxTextureSize_Window   (const iWindow *);
 float       uiScale_Window          (const iWindow *);
 iInt2       coord_Window            (const iWindow *, int x, int y);
 iInt2       mouseCoord_Window       (const iWindow *);
+iAnyObject *hitChild_Window         (const iWindow *, iInt2 coord);
 uint32_t    frameTime_Window        (const iWindow *);
 SDL_Renderer *renderer_Window       (const iWindow *);
 int         snap_Window             (const iWindow *);
 iBool       isFullscreen_Window     (const iWindow *);
+iRoot *     findRoot_Window         (const iWindow *, const iWidget *widget);
 
 iWindow *   get_Window              (void);