overview: Only handled input events in on-screen desktops

kwin does not support true multiscreen drag and drops. Events are sent
to an offscreen location of the screen initiating the drag. Therefore it
is important that off-screen items do not process drop events

BUG: 481331

src/plugins/overview/qml/main.qml

@@ -412,7 +412,11 @@ FocusScope {
             Item {
                 id: mainBackground
 
+                // visible when in the overview 'grid' mode, but also keep neighbouring items visible as they can appear during gestures
                 visible: gridVal > 0 || nearCurrent
+                // Avoid handling drops for offscreen visible items
+                enabled: gridVal > 0 || current
+
                 anchors.fill: parent
                 property bool shouldBeVisibleInOverview: !(effect.searchText.length > 0 && current) || (heap.count !== 0 && effect.filterWindows)
                 opacity: 1 - overviewVal * (shouldBeVisibleInOverview ? 0 : 1)
@@ -566,6 +570,7 @@ FocusScope {
                             if (!mainBackground.contains(mainBackground.mapFromItem(null, pos.x, pos.y))) {
                                 return;
                             }
+                            // moving the window to the client screen is handled by WindowHeap's similiarly named function that is also run
                             item.client.desktops = [mainBackground.desktop];
                         }
                     }