From d7a5dedd4f5e283e21abdf9cddee3100ff91221c Mon Sep 17 00:00:00 2001
From: Alexander Mikhaylenko <alexm@gnome.org>
Date: Mon, 15 Mar 2021 17:17:01 +0500
Subject: [PATCH] scrolledwindow: Cancel overshoot on dimension changes

If we scroll down in a list that's still being filled, we hit the edge and
initiate overshoot, and then the adjustment's upper value increases. This
leads to an unwanted bounce back.

Additionally, if in a similar situation the upper value decreases, the
overscroll glow gets stuck.

Update kinetic scrolling upper and lower value on changes, and immediately
cancel it if dimensions on that side change.

Fixes https://gitlab.gnome.org/GNOME/gtk/-/issues/3752
---
 gtk/gtkkineticscrolling.c        | 29 ++++++++++++++++++++++++
 gtk/gtkkineticscrollingprivate.h | 11 +++++++++
 gtk/gtkscrolledwindow.c          | 39 ++++++++++++++++++++++++++++++++
 3 files changed, 79 insertions(+)

diff --git a/gtk/gtkkineticscrolling.c b/gtk/gtkkineticscrolling.c
index eb9c13bdfd..87618acd7c 100644
--- a/gtk/gtkkineticscrolling.c
+++ b/gtk/gtkkineticscrolling.c
@@ -123,6 +123,35 @@ gtk_kinetic_scrolling_new (double lower,
   return data;
 }
 
+GtkKineticScrollingChange
+gtk_kinetic_scrolling_update_size (GtkKineticScrolling *data,
+                                   double               lower,
+                                   double               upper)
+{
+  GtkKineticScrollingChange change = GTK_KINETIC_SCROLLING_CHANGE_NONE;
+
+  if (lower != data->lower)
+    {
+      if (data->position <= lower)
+        change |= GTK_KINETIC_SCROLLING_CHANGE_LOWER;
+
+      data->lower = lower;
+    }
+
+  if (upper != data->upper)
+    {
+      if (data->position >= data->upper)
+        change |= GTK_KINETIC_SCROLLING_CHANGE_UPPER;
+
+      data->upper = upper;
+    }
+
+  if (data->phase == GTK_KINETIC_SCROLLING_PHASE_OVERSHOOTING)
+    change |= GTK_KINETIC_SCROLLING_CHANGE_IN_OVERSHOOT;
+
+  return change;
+}
+
 void
 gtk_kinetic_scrolling_free (GtkKineticScrolling *kinetic)
 {
diff --git a/gtk/gtkkineticscrollingprivate.h b/gtk/gtkkineticscrollingprivate.h
index b97fac360f..a531ace476 100644
--- a/gtk/gtkkineticscrollingprivate.h
+++ b/gtk/gtkkineticscrollingprivate.h
@@ -23,6 +23,13 @@
 
 G_BEGIN_DECLS
 
+typedef enum {
+  GTK_KINETIC_SCROLLING_CHANGE_NONE = 0,
+  GTK_KINETIC_SCROLLING_CHANGE_LOWER = 1 << 0,
+  GTK_KINETIC_SCROLLING_CHANGE_UPPER = 1 << 1,
+  GTK_KINETIC_SCROLLING_CHANGE_IN_OVERSHOOT = 1 << 2,
+} GtkKineticScrollingChange;
+
 typedef struct _GtkKineticScrolling GtkKineticScrolling;
 
 GtkKineticScrolling *    gtk_kinetic_scrolling_new  (double                lower,
@@ -34,6 +41,10 @@ GtkKineticScrolling *    gtk_kinetic_scrolling_new  (double                lower
                                                      double                initial_velocity);
 void                     gtk_kinetic_scrolling_free (GtkKineticScrolling  *kinetic);
 
+GtkKineticScrollingChange gtk_kinetic_scrolling_update_size (GtkKineticScrolling *data,
+                                                             double               lower,
+                                                             double               upper);
+
 gboolean                 gtk_kinetic_scrolling_tick (GtkKineticScrolling  *data,
                                                      double                time_delta,
                                                      double               *position,
diff --git a/gtk/gtkscrolledwindow.c b/gtk/gtkscrolledwindow.c
index a56891b5e2..40e285c77b 100644
--- a/gtk/gtkscrolledwindow.c
+++ b/gtk/gtkscrolledwindow.c
@@ -3455,6 +3455,24 @@ gtk_scrolled_window_adjustment_changed (GtkAdjustment *adjustment,
 
           if (priv->hscrollbar_visible != visible)
             gtk_widget_queue_resize (GTK_WIDGET (scrolled_window));
+
+          if (priv->hscrolling)
+            {
+              GtkKineticScrollingChange change;
+              double lower = gtk_adjustment_get_lower (adjustment);
+              double upper = gtk_adjustment_get_upper (adjustment);
+              upper -= gtk_adjustment_get_page_size (adjustment);
+
+              change = gtk_kinetic_scrolling_update_size (priv->hscrolling, lower, upper);
+
+              if ((change & GTK_KINETIC_SCROLLING_CHANGE_IN_OVERSHOOT) &&
+                  (change & (GTK_KINETIC_SCROLLING_CHANGE_UPPER | GTK_KINETIC_SCROLLING_CHANGE_LOWER)))
+                {
+                  g_clear_pointer (&priv->hscrolling, gtk_kinetic_scrolling_free);
+                  priv->unclamped_hadj_value = gtk_adjustment_get_value (adjustment);
+                  gtk_scrolled_window_invalidate_overshoot (scrolled_window);
+                }
+            }
         }
     }
   else if (adjustment == gtk_scrollbar_get_adjustment (GTK_SCROLLBAR (priv->vscrollbar)))
@@ -3468,8 +3486,29 @@ gtk_scrolled_window_adjustment_changed (GtkAdjustment *adjustment,
 
           if (priv->vscrollbar_visible != visible)
             gtk_widget_queue_resize (GTK_WIDGET (scrolled_window));
+
+          if (priv->vscrolling)
+            {
+              GtkKineticScrollingChange change;
+              double lower = gtk_adjustment_get_lower (adjustment);
+              double upper = gtk_adjustment_get_upper (adjustment);
+              upper -= gtk_adjustment_get_page_size (adjustment);
+
+              change = gtk_kinetic_scrolling_update_size (priv->vscrolling, lower, upper);
+
+              if ((change & GTK_KINETIC_SCROLLING_CHANGE_IN_OVERSHOOT) &&
+                  (change & (GTK_KINETIC_SCROLLING_CHANGE_UPPER | GTK_KINETIC_SCROLLING_CHANGE_LOWER)))
+                {
+                  g_clear_pointer (&priv->vscrolling, gtk_kinetic_scrolling_free);
+                  priv->unclamped_vadj_value = gtk_adjustment_get_value (adjustment);
+                  gtk_scrolled_window_invalidate_overshoot (scrolled_window);
+                }
+            }
         }
     }
+
+  if (!priv->hscrolling && !priv->vscrolling)
+    gtk_scrolled_window_cancel_deceleration (scrolled_window);
 }
 
 static void
-- 
2.30.2