Fix animation when clicking grouped windows in task manager

Since hidden windows are not placed in the grid-like view, `cell.isReady` is always false for them, and they never switch to the "active-hidden" state when the effect is turned on. This commit makes sure that windows hide even if their cell isn't ready.

src/plugins/private/qml/WindowHeapDelegate.qml

@@ -66,7 +66,7 @@ Item {
         if (thumb.gestureInProgress) {
             return "partial";
         }
-        if (thumb.partialActivationFactor > 0.5 && cell.isReady) {
+        if (thumb.partialActivationFactor > 0.5 && (cell.isReady || activeHidden)) {
             return activeHidden ? "active-hidden" : `active-${substate}`;
         }
         return initialHidden ? "initial-hidden" : "initial";