GDK W32: Handle maximizing correctly for small primary monitors

When primary monitor is smaller than the actual monitor on which the
window is being maximized, the WM will do widnow size adjustments
that will completely screw the window size if we try to make it
smaller than 100% fullscreen (to account for taskbar size, for example).

Fix this by overriding maximized window size during WM_WINDOWPOSCHANGING.

https://bugzilla.gnome.org/show_bug.cgi?id=775808

gdk/win32/gdkevents-win32.c

@@ -2078,6 +2078,25 @@ _gdk_win32_window_fill_min_max_info (GdkWindow  *window,
     }
   else
     {
+      /* According to "How does the window manager adjust ptMaxSize and
+       * ptMaxPosition for multiple monitors?" article
+       * https://blogs.msdn.microsoft.com/oldnewthing/20150501-00/?p=44964
+       * if ptMaxSize >= primary_monitor_size, then it will be adjusted by
+       * WM to account for the monitor size differences if the window gets
+       * maximized on a non-primary monitor, by simply adding the size
+       * difference (i.e. if non-primary monitor is larger by 100px, then
+       * window will be made larger exactly by 100px).
+       * If ptMaxSize < primary_monitor_size at least in one direction,
+       * nothing is adjusted.
+       * Therefore, if primary monitor is smaller than the actual monitor,
+       * then it is not possible to give window a size that is larger than
+       * the primary monitor and smaller than the non-primary monitor,
+       * because WM will always enlarge the window.
+       * Therefore, it is impossible to account for taskbar size.
+       * So we don't try at all. Instead we just remember that we're trying
+       * to maximize the window, catch WM_WINDOWPOSCHANGING and
+       * adjust the size then.
+       */
       HMONITOR nearest_monitor;
       MONITORINFO nearest_info;
 
@@ -3108,6 +3127,10 @@ gdk_event_translate (MSG  *msg,
 	case SC_RESTORE:
 	  do_show_window (window, msg->wParam == SC_MINIMIZE ? TRUE : FALSE);
 	  break;
+        case SC_MAXIMIZE:
+          impl = GDK_WINDOW_IMPL_WIN32 (window->impl);
+          impl->maximizing = TRUE;
+	  break;
 	}
 
       break;
@@ -3159,7 +3182,26 @@ gdk_event_translate (MSG  *msg,
 				  GetNextWindow (msg->hwnd, GW_HWNDPREV))));
 
       if (GDK_WINDOW_IS_MAPPED (window))
-	return_val = ensure_stacking_on_window_pos_changing (msg, window);
+        {
+	  return_val = ensure_stacking_on_window_pos_changing (msg, window);
+
+          impl = GDK_WINDOW_IMPL_WIN32 (window->impl);
+
+          if (impl->maximizing)
+            {
+              MINMAXINFO our_mmi;
+
+              if (_gdk_win32_window_fill_min_max_info (window, &our_mmi))
+                {
+                  windowpos = (WINDOWPOS *) msg->lParam;
+                  windowpos->cx = our_mmi.ptMaxSize.x;
+                  windowpos->cy = our_mmi.ptMaxSize.y;
+                }
+
+              impl->maximizing = FALSE;
+            }
+        }
+
       break;
 
     case WM_WINDOWPOSCHANGED:

gdk/win32/gdkwindow-win32.c

@@ -5677,6 +5677,10 @@ GtkShowWindow (GdkWindow *window,
       break;
     }
 
+  /* Ensure that maximized window size is corrected later on */
+  if (cmd_show == SW_MAXIMIZE)
+    impl->maximizing = TRUE;
+
   return ShowWindow (hwnd, cmd_show);
 }
 

gdk/win32/gdkwindow-win32.h

@@ -285,6 +285,12 @@ struct _GdkWindowImplWin32
    */
   guint have_temp_styles : 1;
 
+  /* If TRUE, the window is in the process of being maximized.
+   * This is set by WM_SYSCOMMAND and by gdk_win32_window_maximize (),
+   * and is unset when WM_WINDOWPOSCHANGING is handled.
+   */
+  guint maximizing : 1;
+
   /* GDK does not keep window contents around, it just draws new
    * stuff over the window where changes occurred.
    * cache_surface retains old window contents, because