notifications: pin hovered popups to their slot

fixes #3758

quickshell/Modules/Notifications/Popup/NotificationPopup.qml

@@ -50,6 +50,7 @@ PanelWindow {
         return false;
     }
     readonly property real screenY: stackMotion.value
+    readonly property real stackTarget: stackMotion.target
     readonly property bool stackMoving: stackMotion.running
     property real presentationProgress: 0
     property real chromeRelease: 0

quickshell/Modules/Notifications/Popup/NotificationPopupManager.qml

@@ -261,7 +261,7 @@ QtObject {
                 return currentY;
             if (!_isLayoutWindow(p))
                 continue;
-            const gap = p.layoutPinned ? Math.max(0, p.screenY - currentY) : 0;
+            const gap = p.layoutPinned ? Math.max(0, p.stackTarget - currentY) : 0;
             currentY += gap + _popupHeight(p);
         }
         return currentY;
@@ -270,7 +270,7 @@ QtObject {
     function _repositionAll() {
         let currentY = topMargin;
         for (const win of _layoutWindows()) {
-            const gap = win.layoutPinned ? Math.max(0, win.screenY - currentY) : 0;
+            const gap = win.layoutPinned ? Math.max(0, win.stackTarget - currentY) : 0;
             const position = currentY + gap;
             win.setStackPosition(position);
             currentY = position + _popupHeight(win);