From 6b2f1ac9c2109a237efff88c544e729445ad09f8 Mon Sep 17 00:00:00 2001
From: Jaakko Keränen <jaakko.keranen@iki.fi>
Date: Mon, 15 Feb 2021 17:50:11 +0200
Subject: DocumentWidget: Scroll position while resizing

Keep the top of the view fixed during horizontal resizing.
---
 res/about/version.gmi   |  3 ++-
 src/ui/documentwidget.c | 54 ++++++++++++++++++++++++++++++++-----------------
 2 files changed, 38 insertions(+), 19 deletions(-)

diff --git a/res/about/version.gmi b/res/about/version.gmi
index dfdf2b97..c255e533 100644
--- a/res/about/version.gmi
+++ b/res/about/version.gmi
@@ -12,7 +12,8 @@
 * Shift+Insert can be used for pasting clipboard contents into input fields.
 * Added keybinding (F11) for toggling fullscreen mode. On macOS, the shortcut is ⌃⌘F as before.
 * Added keybinding for finding text on page.
-* Windows: Added a custom window frame to replace the default Windows one. This looks nicer but does not behave exactly like a native window frame. Added a setting to Preferences > Window for switching back to the default frame.
+* Scroll position remains fixed while horizontally resizing the window or sidebars.
+* Windows: Added a custom window frame to replace the default Windows one. This looks nicer but does not behave exactly like a native window frame. Added a setting to Preferences for switching back to the default frame.
 * Windows: Fixed a flash of white when the window is first opened.
 
 ## 1.1.3
diff --git a/src/ui/documentwidget.c b/src/ui/documentwidget.c
index 603af076..e0415f6c 100644
--- a/src/ui/documentwidget.c
+++ b/src/ui/documentwidget.c
@@ -1113,6 +1113,9 @@ static void scroll_DocumentWidget_(iDocumentWidget *d, int offset) {
 }
 
 static void scrollTo_DocumentWidget_(iDocumentWidget *d, int documentY, iBool centered) {
+    if (!hasSiteBanner_GmDocument(d->doc)) {
+        documentY += d->pageMargin * gap_UI;
+    }
     init_Anim(&d->scrollY,
               documentY - (centered ? documentBounds_DocumentWidget_(d).size.y / 2
                                     : lineHeight_Text(paragraph_FontId)));
@@ -1538,28 +1541,43 @@ static const int homeRowKeys_[] = {
     't', 'y',
 };
 
+static void updateDocumentWidthRetainingScrollPosition_DocumentWidget_(iDocumentWidget *d,
+                                                                       iBool keepCenter) {
+    /* Font changes (i.e., zooming) will keep the view centered, otherwise keep the top
+       of the visible area fixed. */
+    const iGmRun *run     = keepCenter ? middleRun_DocumentWidget_(d) : d->firstVisibleRun;
+    const char *  runLoc  = (run ? run->text.start : NULL);
+    int           voffset = 0;
+    if (!keepCenter && run) {
+        /* Keep the first visible run visible at the same position. */
+        /* TODO: First *fully* visible run? */
+        voffset = visibleRange_DocumentWidget_(d).start - top_Rect(run->visBounds);
+    }
+    setWidth_GmDocument(d->doc, documentWidth_DocumentWidget_(d));
+    if (runLoc && !keepCenter) {
+        run = findRunAtLoc_GmDocument(d->doc, runLoc);
+        if (run) {
+            scrollTo_DocumentWidget_(d,
+                                     top_Rect(run->visBounds) +
+                                         lineHeight_Text(paragraph_FontId) + voffset,
+                                     iFalse);
+        }
+    }
+    else if (runLoc && keepCenter) {
+        run = findRunAtLoc_GmDocument(d->doc, runLoc);
+        if (run) {
+            scrollTo_DocumentWidget_(d, mid_Rect(run->bounds).y, iTrue);
+        }
+    }
+}
+
 static iBool handleCommand_DocumentWidget_(iDocumentWidget *d, const char *cmd) {
     iWidget *w = as_Widget(d);
     if (equal_Command(cmd, "window.resized") || equal_Command(cmd, "font.changed")) {
-        const iBool isVerticalOnly =
-            !argLabel_Command(cmd, "horiz") && argLabel_Command(cmd, "vert");
         /* Alt/Option key may be involved in window size changes. */
         iChangeFlags(d->flags, showLinkNumbers_DocumentWidgetFlag, iFalse);
-        if (isVerticalOnly) {
-            scroll_DocumentWidget_(d, 0); /* prevent overscroll */
-        }
-        else {
-            const iGmRun *mid = middleRun_DocumentWidget_(d);
-            const char *midLoc = (mid ? mid->text.start : NULL);
-            setWidth_GmDocument(d->doc, documentWidth_DocumentWidget_(d));
-            scroll_DocumentWidget_(d, 0);
-            if (midLoc) {
-                mid = findRunAtLoc_GmDocument(d->doc, midLoc);
-                if (mid) {
-                    scrollTo_DocumentWidget_(d, mid_Rect(mid->bounds).y, iTrue);
-                }
-            }
-        }
+        const iBool   keepCenter = equal_Command(cmd, "font.changed");
+        updateDocumentWidthRetainingScrollPosition_DocumentWidget_(d, keepCenter);
         updateSideIconBuf_DocumentWidget_(d);
         updateOutline_DocumentWidget_(d);
         invalidate_DocumentWidget_(d);
@@ -3398,7 +3416,7 @@ iBool isRequestOngoing_DocumentWidget(const iDocumentWidget *d) {
 }
 
 void updateSize_DocumentWidget(iDocumentWidget *d) {
-    setWidth_GmDocument(d->doc, documentWidth_DocumentWidget_(d));
+    updateDocumentWidthRetainingScrollPosition_DocumentWidget_(d, iFalse);
     resetWideRuns_DocumentWidget_(d);
     updateSideIconBuf_DocumentWidget_(d);
     updateOutline_DocumentWidget_(d);
-- 
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