[gtk+] GDK W32: Refuse to release mouse grab while in DnD mode

[Руслан Ижбулатов <ruslanizhb src gnome org>]

commit a66017a6a083dc333d9192c766882c63bcaeffb7
Author: Руслан Ижбулатов <lrn1986 gmail com>
Date:   Thu Sep 14 17:43:38 2017 +0000

    GDK W32: Refuse to release mouse grab while in DnD mode
    
    Handle WM_CANCELMODE and do nothing in response to it when DnD is
    active. Otherwise pass it to DefWindowProc, which will call ReleaseCapture()
    on our behalf.
    This prevents us from losing mouse capture when alt-tabbing during DnD
    (this includes the feature of Windows Explorer where dragging stuff over
    a window button in the taskbar causes that window to receive focus, i.e.
    keyboardless alt-tabbing).
    
    https://bugzilla.gnome.org/show_bug.cgi?id=786509

 gdk/win32/gdkevents-win32.c |   17 +++++++++++++++++
 1 files changed, 17 insertions(+), 0 deletions(-)
---
diff --git a/gdk/win32/gdkevents-win32.c b/gdk/win32/gdkevents-win32.c
index 7b459db..601ca03 100644
--- a/gdk/win32/gdkevents-win32.c
+++ b/gdk/win32/gdkevents-win32.c
@@ -3281,6 +3281,23 @@ gdk_event_translate (MSG  *msg,
 
       break;
 
+    /*
+     * Handle WM_CANCELMODE and do nothing in response to it when DnD is
+     * active. Otherwise pass it to DefWindowProc, which will call ReleaseCapture()
+     * on our behalf.
+     * This prevents us from losing mouse capture when alt-tabbing during DnD
+     * (this includes the feature of Windows Explorer where dragging stuff over
+     * a window button in the taskbar causes that window to receive focus, i.e.
+     * keyboardless alt-tabbing).
+     */
+    case WM_CANCELMODE:
+      if (_modal_operation_in_progress & GDK_WIN32_MODAL_OP_DND)
+        {
+          return_val = TRUE;
+          *ret_valp = 0;
+        }
+      break;
+
     case WM_CAPTURECHANGED:
       /* Sometimes we don't get WM_EXITSIZEMOVE, for instance when you
         select move/size in the menu and then click somewhere without