From d4943ef2fbc5a402bb5f43f40c93700b7263f07b Mon Sep 17 00:00:00 2001
From: Benjamin Schaaf <ben.schaaf@gmail.com>
Date: Fri, 25 Sep 2020 00:32:00 +1000
Subject: Add scaling adjustment for touch dragging a scrolled window

If a GtkScrollable uses a different scale than pixels the offset from a
drag must be adjusted to match this scale. This fixes issues like this
https://gitlab.gnome.org/GNOME/vte/-/issues/283 where the terminal
widget uses rows instead of pixels as its scale for each GtkAdjustable
thus causing a 1 pixel vertical drag to scroll 1 row.
---
 gtk/gtkscrolledwindow.c | 36 +++++++++++++++++++++++++++++++++---
 1 file changed, 33 insertions(+), 3 deletions(-)

diff --git a/gtk/gtkscrolledwindow.c b/gtk/gtkscrolledwindow.c
index 95c910f50e..fa898f14c6 100644
--- a/gtk/gtkscrolledwindow.c
+++ b/gtk/gtkscrolledwindow.c
@@ -1007,7 +1007,9 @@ scrolled_window_drag_update_cb (GtkScrolledWindow *scrolled_window,
   GtkScrolledWindowPrivate *priv = scrolled_window->priv;
   GtkAdjustment *hadjustment;
   GtkAdjustment *vadjustment;
-  gdouble dx, dy;
+  GtkScrollable *child;
+  GtkBorder border;
+  double dx, dy, scale, size;
 
   gtk_scrolled_window_invalidate_overshoot (scrolled_window);
 
@@ -1020,10 +1022,25 @@ scrolled_window_drag_update_cb (GtkScrolledWindow *scrolled_window,
                                       GTK_EVENT_SEQUENCE_CLAIMED);
     }
 
+  child = GTK_SCROLLABLE (gtk_bin_get_child (GTK_BIN (scrolled_window)));
+
   hadjustment = gtk_range_get_adjustment (GTK_RANGE (priv->hscrollbar));
   if (hadjustment && may_hscroll (scrolled_window))
     {
-      dx = priv->drag_start_x - offset_x;
+      if (child && gtk_widget_get_visible (child))
+        {
+          size = gtk_widget_get_allocated_width (GTK_WIDGET (child));
+          if (gtk_scrollable_get_border (child, &border))
+            size -= border.left + border.right;
+
+          scale = gtk_adjustment_get_page_size (hadjustment) / size;
+        }
+      else
+        {
+          scale = 1.0;
+        }
+
+      dx = priv->drag_start_x - offset_x * scale;
       _gtk_scrolled_window_set_adjustment_value (scrolled_window,
                                                  hadjustment, dx);
     }
@@ -1031,7 +1048,20 @@ scrolled_window_drag_update_cb (GtkScrolledWindow *scrolled_window,
   vadjustment = gtk_range_get_adjustment (GTK_RANGE (priv->vscrollbar));
   if (vadjustment && may_vscroll (scrolled_window))
     {
-      dy = priv->drag_start_y - offset_y;
+      if (child && gtk_widget_get_visible (child))
+        {
+          size = gtk_widget_get_allocated_height (GTK_WIDGET (child));
+          if (gtk_scrollable_get_border (child, &border))
+            size -= border.top + border.bottom;
+
+          scale = gtk_adjustment_get_page_size (vadjustment) / size;
+        }
+      else
+        {
+          scale = 1.0;
+        }
+
+      dy = priv->drag_start_y - offset_y * scale;
       _gtk_scrolled_window_set_adjustment_value (scrolled_window,
                                                  vadjustment, dy);
     }
-- 
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