Fix Windows taskbar click not minimizing the active window

src/fe-gtk/maingui.c

@@ -5004,6 +5004,94 @@ mg_tabwindow_de_cb (GtkWidget *widget, GdkEvent *event, gpointer user_data)
 }
 
 #ifdef G_OS_WIN32
+/* GTK3's win32 backend does not minimize the window when its taskbar
+ * button is clicked while the window is active; the shell's minimize
+ * fails and, as the upstream report puts it, "the window only becomes
+ * unfocused but remains in the foreground"
+ * (https://gitlab.gnome.org/GNOME/gtk/-/issues/3749).
+ *
+ * We use exactly that signature to tell the two taskbar gestures apart:
+ * after a taskbar click deactivates the window, look at the foreground
+ * window. If this window is still the foreground window (the shell tried
+ * to minimize it and failed), the click was on its own button, so
+ * minimize it manually. If activation moved to the taskbar, the Start
+ * menu, another application or one of our other windows, the user was
+ * not clicking this window's button, so leave it alone.
+ */
+#define MG_WIN32_TASKBAR_MINIMIZE_DELAY_MS 200
+
+static guint mg_win32_taskbar_minimize_source_id = 0;
+static HWND mg_win32_taskbar_minimize_hwnd = NULL;
+
+static gboolean
+mg_win32_hwnd_is_taskbar (HWND hwnd)
+{
+	wchar_t class_name[32];
+	HWND root;
+
+	if (!hwnd)
+		return FALSE;
+
+	root = GetAncestor (hwnd, GA_ROOT);
+	if (!root)
+		root = hwnd;
+
+	if (GetClassNameW (root, class_name, G_N_ELEMENTS (class_name)) == 0)
+		return FALSE;
+
+	return lstrcmpW (class_name, L"Shell_TrayWnd") == 0
+		|| lstrcmpW (class_name, L"Shell_SecondaryTrayWnd") == 0;
+}
+
+static gboolean
+mg_win32_taskbar_minimize_cb (gpointer userdata)
+{
+	HWND hwnd = mg_win32_taskbar_minimize_hwnd;
+	HWND foreground;
+
+	mg_win32_taskbar_minimize_source_id = 0;
+	mg_win32_taskbar_minimize_hwnd = NULL;
+
+	if (!hwnd || !IsWindow (hwnd) || !IsWindowVisible (hwnd) || IsIconic (hwnd))
+		return G_SOURCE_REMOVE;
+
+	foreground = GetForegroundWindow ();
+
+	/* Activation moved to another window (another application, the
+	 * taskbar itself for an empty-space/Start/tray click, or one of our
+	 * other windows), so the taskbar click was not on this window's own
+	 * button - leave it alone. Only the failed shell-minimize leaves this
+	 * window still foreground (or leaves nothing foreground at all). */
+	if (foreground != NULL && foreground != hwnd)
+		return G_SOURCE_REMOVE;
+
+	PostMessage (hwnd, WM_SYSCOMMAND, SC_MINIMIZE, 0);
+	return G_SOURCE_REMOVE;
+}
+
+static void
+mg_win32_taskbar_click_check (HWND hwnd)
+{
+	POINT cursor_pos;
+
+	if (!IsWindowVisible (hwnd) || IsIconic (hwnd))
+		return;
+
+	if (!GetCursorPos (&cursor_pos))
+		return;
+
+	if (!mg_win32_hwnd_is_taskbar (WindowFromPoint (cursor_pos)))
+		return;
+
+	if (mg_win32_taskbar_minimize_source_id)
+		g_source_remove (mg_win32_taskbar_minimize_source_id);
+
+	mg_win32_taskbar_minimize_hwnd = hwnd;
+	mg_win32_taskbar_minimize_source_id =
+		g_timeout_add (MG_WIN32_TASKBAR_MINIMIZE_DELAY_MS,
+		               mg_win32_taskbar_minimize_cb, NULL);
+}
+
 static GdkFilterReturn
 mg_win32_filter (GdkXEvent *xevent, GdkEvent *event, gpointer data)
 {
@@ -5012,6 +5100,13 @@ mg_win32_filter (GdkXEvent *xevent, GdkEvent *event, gpointer data)
 	if (!msg)
 		return GDK_FILTER_CONTINUE;
 
+	if (msg->message == WM_ACTIVATE)
+	{
+		if (LOWORD (msg->wParam) == WA_INACTIVE)
+			mg_win32_taskbar_click_check (msg->hwnd);
+		return GDK_FILTER_CONTINUE;
+	}
+
 	if (msg->message == WM_TIMECHANGE)
 	{
 		_tzset();