[gimp] app: fix render cache artifacts when resizing the canvas

[Michael Natterer <mitch src gnome org>]

commit d88b90d026302fdbc7be2f7885e19f0085f63ccd
Author: Michael Natterer <mitch gimp org>
Date:   Sun Jul 21 18:13:53 2019 +0200

    app: fix render cache artifacts when resizing the canvas
    
    Since introducing gimp_display_shell_canvas_tick() we were setting
    shell->disp_width and shell->disp_height in the tick callback, which
    was too late and caused redundant rendering and scrolling, but was
    never noticed.
    
    Now we clear the render cache and its valid region in
    gimp_display_shell_canvas_size_allocate() directly and set
    shell->disp_width and shell->disp_height immediately, so other places
    that listen to the canvas' size-allocate get the right values.
    
    The old size of the canvas gets to the tick callback using a small
    struct as user data.

 app/display/gimpdisplayshell-callbacks.c | 60 ++++++++++++++++++++++----------
 1 file changed, 41 insertions(+), 19 deletions(-)
---
diff --git a/app/display/gimpdisplayshell-callbacks.c b/app/display/gimpdisplayshell-callbacks.c
index 3875e4902e..e40bc27100 100644
--- a/app/display/gimpdisplayshell-callbacks.c
+++ b/app/display/gimpdisplayshell-callbacks.c
@@ -116,26 +116,31 @@ gimp_display_shell_canvas_realize (GtkWidget        *canvas,
   gtk_widget_set_size_request (GTK_WIDGET (shell), 0, 0);
 }
 
+typedef struct
+{
+  GimpDisplayShell *shell;
+  gint              prev_width;
+  gint              prev_height;
+} TickClosure;
+
 static gboolean
-gimp_display_shell_canvas_tick (GtkWidget        *widget,
-                                GdkFrameClock    *frame_clock,
-                                GimpDisplayShell *shell)
+gimp_display_shell_canvas_tick (GtkWidget     *widget,
+                                GdkFrameClock *frame_clock,
+                                TickClosure   *tick)
 {
-  GtkAllocation allocation;
+  GimpDisplayShell *shell = tick->shell;
+  GtkAllocation     allocation;
 
   gtk_widget_get_allocation (widget, &allocation);
 
-  if ((shell->disp_width  != allocation.width) ||
-      (shell->disp_height != allocation.height))
+  if ((tick->prev_width  != allocation.width) ||
+      (tick->prev_height != allocation.height))
     {
-      g_clear_pointer (&shell->render_cache, cairo_surface_destroy);
-      gimp_display_shell_render_invalidate_full (shell);
-
       if (shell->zoom_on_resize   &&
-          shell->disp_width  > 64 &&
-          shell->disp_height > 64 &&
-          allocation.width   > 64 &&
-          allocation.height  > 64)
+          tick->prev_width  > 64 &&
+          tick->prev_height > 64 &&
+          allocation.width  > 64 &&
+          allocation.height > 64)
         {
           gdouble scale = gimp_zoom_model_get_factor (shell->zoom);
           gint    offset_x;
@@ -148,8 +153,8 @@ gimp_display_shell_canvas_tick (GtkWidget        *widget,
            */
           scale *= (sqrt (SQR (allocation.width) +
                           SQR (allocation.height)) /
-                    sqrt (SQR (shell->disp_width) +
-                          SQR (shell->disp_height)));
+                    sqrt (SQR (tick->prev_width) +
+                          SQR (tick->prev_height)));
 
           offset_x = UNSCALEX (shell, shell->offset_x);
           offset_y = UNSCALEX (shell, shell->offset_y);
@@ -160,9 +165,6 @@ gimp_display_shell_canvas_tick (GtkWidget        *widget,
           shell->offset_y = SCALEY (shell, offset_y);
         }
 
-      shell->disp_width  = allocation.width;
-      shell->disp_height = allocation.height;
-
       /* When we size-allocate due to resize of the top level window,
        * we want some additional logic. Don't apply it on
        * zoom_on_resize though.
@@ -224,6 +226,8 @@ gimp_display_shell_canvas_tick (GtkWidget        *widget,
   /* undo size request from gimp_display_shell_constructed() */
   gtk_widget_set_size_request (widget, -1, -1);
 
+  g_free (tick);
+
   return G_SOURCE_REMOVE;
 }
 
@@ -232,13 +236,31 @@ gimp_display_shell_canvas_size_allocate (GtkWidget        *widget,
                                          GtkAllocation    *allocation,
                                          GimpDisplayShell *shell)
 {
+  TickClosure *tick;
+
   /*  are we in destruction?  */
   if (! shell->display || ! gimp_display_get_shell (shell->display))
     return;
 
+  tick = g_new0 (TickClosure, 1);
+
+  tick->shell       = shell;
+  tick->prev_width  = shell->disp_width;
+  tick->prev_height = shell->disp_height;
+
+  if (shell->disp_width  != allocation->width ||
+      shell->disp_height != allocation->height)
+    {
+      g_clear_pointer (&shell->render_cache, cairo_surface_destroy);
+      gimp_display_shell_render_invalidate_full (shell);
+
+      shell->disp_width  = allocation->width;
+      shell->disp_height = allocation->height;
+    }
+
   gtk_widget_add_tick_callback (widget,
                                 (GtkTickCallback) gimp_display_shell_canvas_tick,
-                                shell, NULL);
+                                tick, NULL);
 }
 
 gboolean