xwayland: Add heuristic for WM windows based on reparenting

If the WM client reparents a window, mark the new parent as a WM window.

This helps with current mutter, where decoration windows are created by
a separate mutter-x11-frames client instead of the WM client. There
might be other compositors doing something similar now or in the future.

v2:
* Skip the whole unwrap-and-call-down dance in xwl_reparent_window if
  xwl_screen->ReparentWindow is NULL.

Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/2069>

hw/xwayland/xwayland-screen.c

@@ -1152,6 +1152,9 @@ xwl_screen_init(ScreenPtr pScreen, int argc, char **argv)
     xwl_screen->ConfigNotify = pScreen->ConfigNotify;
     pScreen->ConfigNotify = xwl_config_notify;
 
+    xwl_screen->ReparentWindow = pScreen->ReparentWindow;
+    pScreen->ReparentWindow = xwl_reparent_window;
+
     xwl_screen->ResizeWindow = pScreen->ResizeWindow;
     pScreen->ResizeWindow = xwl_resize_window;
 

hw/xwayland/xwayland-screen.h

@@ -78,6 +78,7 @@ struct xwl_screen {
     XYToWindowProcPtr XYToWindow;
     SetWindowPixmapProcPtr SetWindowPixmap;
     ChangeWindowAttributesProcPtr ChangeWindowAttributes;
+    ReparentWindowProcPtr ReparentWindow;
     ResizeWindowProcPtr ResizeWindow;
     MoveWindowProcPtr MoveWindow;
     SourceValidateProcPtr SourceValidate;

hw/xwayland/xwayland-window.c

@@ -68,6 +68,7 @@
 #define FRACTIONAL_SCALE_DENOMINATOR 120
 
 static DevPrivateKeyRec xwl_window_private_key;
+static DevPrivateKeyRec xwl_wm_window_private_key;
 static DevPrivateKeyRec xwl_damage_private_key;
 static const char *xwl_surface_tag = "xwl-surface";
 
@@ -474,8 +475,13 @@ static Bool
 window_is_wm_window(WindowPtr window)
 {
     struct xwl_screen *xwl_screen = xwl_screen_get(window->drawable.pScreen);
+    Bool *is_wm_window;
 
-    return CLIENT_ID(window->drawable.id) == xwl_screen->wm_client_id;
+    if (CLIENT_ID(window->drawable.id) == xwl_screen->wm_client_id)
+        return TRUE;
+
+    is_wm_window = dixLookupPrivate(&window->devPrivates, &xwl_wm_window_private_key);
+    return *is_wm_window;
 }
 
 static WindowPtr
@@ -1844,6 +1850,31 @@ xwl_config_notify(WindowPtr window,
     return ret;
 }
 
+void
+xwl_reparent_window(WindowPtr window, WindowPtr prior_parent)
+{
+    ScreenPtr screen = window->drawable.pScreen;
+    struct xwl_screen *xwl_screen = xwl_screen_get(screen);
+    WindowPtr parent = window->parent;
+    Bool *is_wm_window;
+
+    if (xwl_screen->ReparentWindow) {
+        screen->ReparentWindow = xwl_screen->ReparentWindow;
+        screen->ReparentWindow(window, prior_parent);
+        xwl_screen->ReparentWindow = screen->ReparentWindow;
+        screen->ReparentWindow = xwl_reparent_window;
+    }
+
+    if (!parent->parent ||
+        GetCurrentClient()->index != xwl_screen->wm_client_id)
+        return;
+
+    /* If the WM client reparents a window, mark the new parent as a WM window */
+    is_wm_window = dixLookupPrivate(&parent->devPrivates,
+                                    &xwl_wm_window_private_key);
+    *is_wm_window = TRUE;
+}
+
 void
 xwl_resize_window(WindowPtr window,
                   int x, int y,
@@ -2068,6 +2099,10 @@ xwl_window_init(void)
     if (!dixRegisterPrivateKey(&xwl_window_private_key, PRIVATE_WINDOW, 0))
         return FALSE;
 
+    if (!dixRegisterPrivateKey(&xwl_wm_window_private_key, PRIVATE_WINDOW,
+                               sizeof(Bool)))
+        return FALSE;
+
     if (!dixRegisterPrivateKey(&xwl_damage_private_key, PRIVATE_WINDOW, 0))
         return FALSE;
 

hw/xwayland/xwayland-window.h

@@ -138,6 +138,7 @@ int xwl_config_notify(WindowPtr window,
                       int x, int y,
                       int width, int height, int bw,
                       WindowPtr sib);
+void xwl_reparent_window(WindowPtr window, WindowPtr prior_parent);
 void xwl_resize_window(WindowPtr window,
                        int x, int y,
                        unsigned int width, unsigned int height,