[gimp] Bug 796252 - Mouse wheel zooming should center on cursor...

[Michael Natterer <mitch src gnome org>]

commit ef2cf21f109007e722138feb2f945688c535085f
Author: Michael Natterer <mitch gimp org>
Date:   Sat May 19 14:31:01 2018 +0200

    Bug 796252 - Mouse wheel zooming should center on cursor...
    
    ... _even at low zoom levels_
    
    Pass GIMP_ZOOM_FOCUS_POINTER to gimp_display_shell_scale() when
    wheel-scrolling, and change the scaling code to really honor
    GIMP_ZOOM_FOCUS_POINTER and not apply magic image centering.
    
    This keep the same point centered under the mouse for wheel-scrolling
    and the zoom tool (== when the zooming is really triggered at a
    certain mouse position).

 NEWS                                       |    3 +
 app/display/gimpdisplayshell-scale.c       |   72 +++++++++++++++-------------
 app/display/gimpdisplayshell-tool-events.c |    4 +-
 3 files changed, 43 insertions(+), 36 deletions(-)
---
diff --git a/NEWS b/NEWS
index e0c4509..00cd086 100644
--- a/NEWS
+++ b/NEWS
@@ -43,6 +43,9 @@ Usability:
   - Mention pressing Enter to complete Foreground selection in the status bar.
   - Make sure that clicking outside any selection prevents Rectangle Select
     and Ellipse Select tools from creating empty selections.
+  - Improve mouse wheel zooming behavior to keep the same point
+    centered under the pointer even if the image is completely visible
+    in the canvas.
 
 Translations:
 
diff --git a/app/display/gimpdisplayshell-scale.c b/app/display/gimpdisplayshell-scale.c
index 17eb0d4..ffbb83c 100644
--- a/app/display/gimpdisplayshell-scale.c
+++ b/app/display/gimpdisplayshell-scale.c
@@ -351,7 +351,9 @@ gimp_display_shell_scale (GimpDisplayShell *shell,
                           gdouble           new_scale,
                           GimpZoomFocus     zoom_focus)
 {
-  gdouble current_scale;
+  GimpDisplayConfig *config;
+  gdouble            current_scale;
+  gboolean           resize_window;
 
   g_return_if_fail (GIMP_IS_DISPLAY_SHELL (shell));
   g_return_if_fail (shell->canvas != NULL);
@@ -361,25 +363,44 @@ gimp_display_shell_scale (GimpDisplayShell *shell,
   if (zoom_type != GIMP_ZOOM_TO)
     new_scale = gimp_zoom_model_zoom_step (zoom_type, current_scale);
 
-  if (! SCALE_EQUALS (new_scale, current_scale))
-    {
-      GimpDisplayConfig *config = shell->display->config;
-      gboolean           resize_window;
+  if (SCALE_EQUALS (new_scale, current_scale))
+    return;
 
-      /* Resize windows only in multi-window mode */
-      resize_window = (config->resize_windows_on_zoom &&
-                       ! GIMP_GUI_CONFIG (config)->single_window_mode);
+  config = shell->display->config;
 
-      if (resize_window)
-        {
-          /* If the window is resized on zoom, simply do the zoom and
-           * get things rolling
-           */
-          gimp_zoom_model_zoom (shell->zoom, GIMP_ZOOM_TO, new_scale);
+  /* Resize windows only in multi-window mode */
+  resize_window = (config->resize_windows_on_zoom &&
+                   ! GIMP_GUI_CONFIG (config)->single_window_mode);
 
-          gimp_display_shell_scale_resize (shell, TRUE, FALSE);
-        }
-      else
+  if (resize_window)
+    {
+      /* If the window is resized on zoom, simply do the zoom and get
+       * things rolling
+       */
+      gimp_zoom_model_zoom (shell->zoom, GIMP_ZOOM_TO, new_scale);
+
+      gimp_display_shell_scale_resize (shell, TRUE, FALSE);
+    }
+  else
+    {
+      gdouble  x, y;
+      gint     image_center_x;
+      gint     image_center_y;
+
+      gimp_display_shell_scale_get_zoom_focus (shell,
+                                               new_scale,
+                                               current_scale,
+                                               &x,
+                                               &y,
+                                               zoom_focus);
+      gimp_display_shell_scale_get_image_center_viewport (shell,
+                                                          &image_center_x,
+                                                          &image_center_y);
+
+      gimp_display_shell_scale_to (shell, new_scale, x, y);
+
+      /* skip centering magic if pointer focus was requested */
+      if (zoom_focus != GIMP_ZOOM_FOCUS_POINTER)
         {
           gboolean starts_fitting_horiz;
           gboolean starts_fitting_vert;
@@ -387,22 +408,6 @@ gimp_display_shell_scale (GimpDisplayShell *shell,
           gboolean zoom_focus_almost_centered_vert;
           gboolean image_center_almost_centered_horiz;
           gboolean image_center_almost_centered_vert;
-          gdouble  x, y;
-          gint     image_center_x;
-          gint     image_center_y;
-
-          gimp_display_shell_scale_get_zoom_focus (shell,
-                                                   new_scale,
-                                                   current_scale,
-                                                   &x,
-                                                   &y,
-                                                   zoom_focus);
-          gimp_display_shell_scale_get_image_center_viewport (shell,
-                                                              &image_center_x,
-                                                              &image_center_y);
-
-          gimp_display_shell_scale_to (shell, new_scale, x, y);
-
 
           /* If an image axis started to fit due to zooming out or if
            * the focus point is as good as in the center, center on
@@ -1218,7 +1223,6 @@ gimp_display_shell_scale_get_zoom_focus (GimpDisplayShell *shell,
         }
     }
 
-  /* Decide which one to use for each axis */
   if (zoom_focus == GIMP_ZOOM_FOCUS_RETAIN_CENTERING_ELSE_BEST_GUESS)
     {
       if (gimp_display_shell_scale_viewport_coord_almost_centered (shell,
diff --git a/app/display/gimpdisplayshell-tool-events.c b/app/display/gimpdisplayshell-tool-events.c
index 73e612d..95b55f5 100644
--- a/app/display/gimpdisplayshell-tool-events.c
+++ b/app/display/gimpdisplayshell-tool-events.c
@@ -944,14 +944,14 @@ gimp_display_shell_canvas_tool_events_internal (GtkWidget         *canvas,
                     gimp_display_shell_scale (shell,
                                               GIMP_ZOOM_IN,
                                               0.0,
-                                              GIMP_ZOOM_FOCUS_BEST_GUESS);
+                                              GIMP_ZOOM_FOCUS_POINTER);
                     break;
 
                   case GDK_SCROLL_DOWN:
                     gimp_display_shell_scale (shell,
                                               GIMP_ZOOM_OUT,
                                               0.0,
-                                              GIMP_ZOOM_FOCUS_BEST_GUESS);
+                                              GIMP_ZOOM_FOCUS_POINTER);
                     break;
 
                   default: