[gtk: 1/2] GdkWin32: Fix mouse events in presence of transparent windows on the desktop

[Matthias Clasen <matthiasc src gnome org>]

commit aec87a3514616ab83b698b85490357b7eeca4d56
Author: Luca Bacci <luca bacci982 gmail com>
Date:   Fri Nov 6 22:43:45 2020 +0100

    GdkWin32: Fix mouse events in presence of transparent windows on the desktop
    
    In gdkdevice-win32.c we are interested in knowing which window
    receives mouse input at a specific location.
    
    Only WindowFromPoint is the right API for the task, other API's
    (such as (Real)ChildWindowFromPoint(Ex)) have shortcomings because
    they are really designed for other purposes. For example, only
    WindowFromPoint is able to look through transparent layered windows.
    
    So even if we want to find a direct child we have to use
    WindowFromPoint and then walk up the hierarchy.
    
    Fixes: #370, #417
    See: !2800

 gdk/win32/gdkdevice-win32.c | 47 ++++++++++++++++-----------------------------
 1 file changed, 17 insertions(+), 30 deletions(-)
---
diff --git a/gdk/win32/gdkdevice-win32.c b/gdk/win32/gdkdevice-win32.c
index 90fae48ac1..35802b93a2 100644
--- a/gdk/win32/gdkdevice-win32.c
+++ b/gdk/win32/gdkdevice-win32.c
@@ -174,38 +174,25 @@ _gdk_device_win32_surface_at_position (GdkDevice       *device,
   GdkSurface *window = NULL;
   GdkWin32Surface *impl = NULL;
   POINT screen_pt, client_pt;
-  HWND hwnd, hwndc;
+  HWND hwnd;
   RECT rect;
 
-  GetCursorPos (&screen_pt);
-
-  /* Only consider visible children of the desktop to avoid the various
-   * non-visible windows you often find on a running Windows box. These
-   * might overlap our windows and cause our walk to fail. As we assume
-   * WindowFromPoint() can find our windows, we follow similar logic
-   * here, and ignore invisible and disabled windows.
-   */
-  hwnd = GetDesktopWindow ();
-  do {
-    window = gdk_win32_handle_table_lookup (hwnd);
-
-    if (window != NULL)
-      break;
-
-    screen_to_client (hwnd, screen_pt, &client_pt);
-    hwndc = ChildWindowFromPointEx (hwnd, client_pt, CWP_SKIPDISABLED  |
-                                                    CWP_SKIPINVISIBLE);
-
-    /* Verify that we're really inside the client area of the window */
-    if (hwndc != hwnd)
-      {
-       GetClientRect (hwndc, &rect);
-       screen_to_client (hwndc, screen_pt, &client_pt);
-       if (!PtInRect (&rect, client_pt))
-         hwndc = hwnd;
-      }
-
-  } while (hwndc != hwnd && (hwnd = hwndc, 1));
+  if (!GetCursorPos (&screen_pt))
+    return NULL;
+
+  /* Use WindowFromPoint instead of ChildWindowFromPoint(Ex).
+  *  Only WindowFromPoint is able to look through transparent
+  *  layered windows.
+  */
+  hwnd = GetAncestor (WindowFromPoint (screen_pt), GA_ROOT);
+
+  /* Verify that we're really inside the client area of the window */
+  GetClientRect (hwnd, &rect);
+  screen_to_client (hwnd, screen_pt, &client_pt);
+  if (!PtInRect (&rect, client_pt))
+    hwnd = NULL;
+
+  window = gdk_win32_handle_table_lookup (hwnd);
 
   if (window && (win_x || win_y))
     {