[gimp/gimp-2-10] Issue #1531 - Zooming with mouse movement should keep track of original point

[Michael Natterer <mitch src gnome org>]

commit f6766cbc876189469047db794e73ab32e3d17774
Author: Michael Natterer <mitch gimp org>
Date:   Mon Jun 4 11:42:02 2018 +0200

    Issue #1531 - Zooming with mouse movement should keep track of original point
    
    When Control-Button2-Zooming, remember the start point, pass it to
    gimp_display_shell_scale_drag() and force gimp_display_shell_scale()
    to zoom around that point by passing GIMP_ZOOM_FOCUS_POINTER and
    faking the point using gimp_display_shell_push_zoom_focus_pointer_pos().
    
    (cherry picked from commit 792cd581a2157084f384d9e5f0a0e84b4ddd74fe)

 app/display/gimpdisplayshell-scale.c       | 8 ++++++--
 app/display/gimpdisplayshell-scale.h       | 2 ++
 app/display/gimpdisplayshell-tool-events.c | 6 ++++++
 app/display/gimpdisplayshell.h             | 2 ++
 4 files changed, 16 insertions(+), 2 deletions(-)
---
diff --git a/app/display/gimpdisplayshell-scale.c b/app/display/gimpdisplayshell-scale.c
index ffbb83ce59..7a5a7a6dc8 100644
--- a/app/display/gimpdisplayshell-scale.c
+++ b/app/display/gimpdisplayshell-scale.c
@@ -635,6 +635,8 @@ gimp_display_shell_scale_by_values (GimpDisplayShell *shell,
 
 void
 gimp_display_shell_scale_drag (GimpDisplayShell *shell,
+                               gdouble           start_x,
+                               gdouble           start_y,
                                gdouble           delta_x,
                                gdouble           delta_y)
 {
@@ -644,19 +646,21 @@ gimp_display_shell_scale_drag (GimpDisplayShell *shell,
 
   scale = gimp_zoom_model_get_factor (shell->zoom);
 
+  gimp_display_shell_push_zoom_focus_pointer_pos (shell, start_x, start_y);
+
   if (delta_y > 0)
     {
       gimp_display_shell_scale (shell,
                                 GIMP_ZOOM_TO,
                                 scale * 1.1,
-                                GIMP_ZOOM_FOCUS_RETAIN_CENTERING_ELSE_BEST_GUESS);
+                                GIMP_ZOOM_FOCUS_POINTER);
     }
   else if (delta_y < 0)
     {
       gimp_display_shell_scale (shell,
                                 GIMP_ZOOM_TO,
                                 scale * 0.9,
-                                GIMP_ZOOM_FOCUS_RETAIN_CENTERING_ELSE_BEST_GUESS);
+                                GIMP_ZOOM_FOCUS_POINTER);
     }
 }
 
diff --git a/app/display/gimpdisplayshell-scale.h b/app/display/gimpdisplayshell-scale.h
index bb1f0ac7b1..530f0e8bef 100644
--- a/app/display/gimpdisplayshell-scale.h
+++ b/app/display/gimpdisplayshell-scale.h
@@ -61,6 +61,8 @@ void     gimp_display_shell_scale_by_values          (GimpDisplayShell *shell,
                                                       gboolean          resize_window);
 
 void     gimp_display_shell_scale_drag               (GimpDisplayShell *shell,
+                                                      gdouble           start_x,
+                                                      gdouble           start_y,
                                                       gdouble           delta_x,
                                                       gdouble           delta_y);
 
diff --git a/app/display/gimpdisplayshell-tool-events.c b/app/display/gimpdisplayshell-tool-events.c
index 95b55f5355..7ca55ab74b 100644
--- a/app/display/gimpdisplayshell-tool-events.c
+++ b/app/display/gimpdisplayshell-tool-events.c
@@ -1530,6 +1530,8 @@ gimp_display_shell_start_scrolling (GimpDisplayShell *shell,
   gimp_display_shell_pointer_grab (shell, event, GDK_POINTER_MOTION_MASK);
 
   shell->scrolling         = TRUE;
+  shell->scroll_start_x    = x;
+  shell->scroll_start_y    = y;
   shell->scroll_last_x     = x;
   shell->scroll_last_y     = y;
   shell->rotating          = (state & gimp_get_extend_selection_mask ()) ? TRUE : FALSE;
@@ -1556,6 +1558,8 @@ gimp_display_shell_stop_scrolling (GimpDisplayShell *shell,
   gimp_display_shell_unset_override_cursor (shell);
 
   shell->scrolling         = FALSE;
+  shell->scroll_start_x    = 0;
+  shell->scroll_start_y    = 0;
   shell->scroll_last_x     = 0;
   shell->scroll_last_y     = 0;
   shell->rotating          = FALSE;
@@ -1591,6 +1595,8 @@ gimp_display_shell_handle_scrolling (GimpDisplayShell *shell,
   else if (shell->scaling)
     {
       gimp_display_shell_scale_drag (shell,
+                                     shell->scroll_start_x,
+                                     shell->scroll_start_y,
                                      shell->scroll_last_x - x,
                                      shell->scroll_last_y - y);
     }
diff --git a/app/display/gimpdisplayshell.h b/app/display/gimpdisplayshell.h
index b00f975f04..e1018338c3 100644
--- a/app/display/gimpdisplayshell.h
+++ b/app/display/gimpdisplayshell.h
@@ -199,6 +199,8 @@ struct _GimpDisplayShell
   const gchar       *space_shaded_tool;
 
   gboolean           scrolling;
+  gint               scroll_start_x;
+  gint               scroll_start_y;
   gint               scroll_last_x;
   gint               scroll_last_y;
   gboolean           rotating;