The initial Slint problem was larger than thought. Hopefully a more universal fix.

internal/platform/xorg/commands_native.c

@@ -175,6 +175,57 @@ PointerPosition cthulhu_read_pointer(Display *display) {
     if (cthulhu_x11_end_read(display)) result = (PointerPosition){0};
     return result;
 }
+// Resolve only the client's immediate enclosing frame. Some WMs omit or leave
+// stale _NET_FRAME_EXTENTS, while GTK reports the real reparenting frame.
+// A container with another managed client is not this client's decoration.
+// The caller holds the Xlib error guard throughout these read-only queries.
+static void cthulhu_read_frame_bounds(Display *display, Window window, Foreground *result) {
+    Window root, parent, *children = NULL;
+    unsigned int count = 0;
+    if (!XQueryTree(display, window, &root, &parent, &children, &count)) return;
+    if (children) XFree(children);
+    if (parent == None || parent == root) return;
+
+    XWindowAttributes attributes;
+    if (!XGetWindowAttributes(display, parent, &attributes) ||
+        attributes.class != InputOutput || attributes.override_redirect ||
+        attributes.map_state != IsViewable) return;
+    Atom stateAtom = XInternAtom(display, "WM_STATE", False);
+    Atom type = None; int format = 0; unsigned long items = 0;
+    unsigned char *data = cthulhu_window_property(display, parent, stateAtom, &type, &format, &items);
+    if (data) XFree(data);
+    if (type != None) return;
+
+    Window treeRoot, frameParent;
+    children = NULL;
+    if (!XQueryTree(display, parent, &treeRoot, &frameParent, &children, &count)) return;
+    int unambiguous = count <= 64, foundClient = 0;
+    for (unsigned int index = 0; unambiguous && index < count; index++) {
+        if (children[index] == window) { foundClient = 1; continue; }
+        data = cthulhu_window_property(display, children[index], stateAtom, &type, &format, &items);
+        if (data) XFree(data);
+        if (type != None) unambiguous = 0;
+    }
+    if (children) XFree(children);
+    if (!unambiguous || !foundClient) return;
+
+    int x, y;
+    Window child;
+    if (!XTranslateCoordinates(display, parent, root, 0, 0, &x, &y, &child)) return;
+    int border = attributes.border_width;
+    x -= border; y -= border;
+    int width = attributes.width + 2 * border, height = attributes.height + 2 * border;
+    // Decorations must enclose the client and remain bounded like the EWMH
+    // extents. Never turn an unrelated workspace into a frame match.
+    int left = result->x - x, top = result->y - y;
+    int right = x + width - result->x - result->width;
+    int bottom = y + height - result->y - result->height;
+    if (left < 0 || top < 0 || right < 0 || bottom < 0 ||
+        left >= 1024 || top >= 1024 || right >= 1024 || bottom >= 1024) return;
+    result->frameX = x; result->frameY = y;
+    result->frameWidth = width; result->frameHeight = height;
+}
+
 // Read-only metadata connection, owned by the caller's native event thread.
 // All requests which can race window destruction share the Xlib error guard.
 Foreground read_foreground(Display *display) {
@@ -227,6 +278,7 @@ Foreground read_foreground(Display *display) {
                 }
             }
             if (data) XFree(data);
+            cthulhu_read_frame_bounds(display, window, &result);
             data=cthulhu_window_property(display,window,XInternAtom(display,"_NET_WM_PID",False),&type,&format,&items);
             if (data && type==XA_CARDINAL && format==32 && items==1) result.pid=(int)((unsigned long *)data)[0];
             if (data) XFree(data);