Fix picking drag target

The normal stacking order contains managed and unmanaged windows now.
This kind of breaks dragging from X11 to Wayland clients.

When an X11 client is the drag source, it's going to create an override
redirect window for the drag icon, which is beneath the pointer.

findManagedToplevel() will return the drag icon instead of the window
beneath the drag icon.

The root problem is that normal event processing has different needs. It
doesn't care about window type except when the screen is locked, while
dnd does. Perhaps it also makes sense to check whether the window accepts
focus.

This change introduces a specialized helper to pick the drag target.
Unfortunately, it amounts to some code duplication. On the other hand,
not sure that other options (such as adding flags to findToplevel()) are
better.

src/input.cpp

@@ -2398,10 +2398,7 @@ public:
             seat->notifyPointerMotion(pos);
             seat->notifyPointerFrame();
 
-            const auto eventPos = event->globalPos();
-            // TODO: use InputDeviceHandler::at() here and check isClient()?
-            Window *t = input()->findManagedToplevel(eventPos);
-            const auto dragTarget = static_cast<Window *>(t && t->isClient() ? t : nullptr);
+            Window *dragTarget = pickDragTarget(pos);
             if (dragTarget) {
                 if (dragTarget != m_dragTarget) {
                     workspace()->takeActivity(dragTarget, Workspace::ActivityFlag::ActivityFocus);
@@ -2416,7 +2413,7 @@ public:
             m_dragTarget = dragTarget;
 
             if (auto *xwl = kwinApp()->xwayland()) {
-                const auto ret = xwl->dragMoveFilter(t, eventPos);
+                const auto ret = xwl->dragMoveFilter(dragTarget, event->globalPos());
                 if (ret == Xwl::DragEventReply::Ignore) {
                     return false;
                 } else if (ret == Xwl::DragEventReply::Take) {
@@ -2424,10 +2421,10 @@ public:
                 }
             }
 
-            if (t) {
+            if (dragTarget) {
                 // TODO: consider decorations
-                if (t->surface() != seat->dragSurface()) {
-                    seat->setDragTarget(dropHandler(t), t->surface(), t->inputTransformation());
+                if (dragTarget->surface() != seat->dragSurface()) {
+                    seat->setDragTarget(dropHandler(dragTarget), dragTarget->surface(), dragTarget->inputTransformation());
                 }
             } else {
                 // no window at that place, if we have a surface we need to reset
@@ -2491,7 +2488,7 @@ public:
         seat->setTimestamp(time);
         seat->notifyTouchMotion(id, pos);
 
-        if (Window *t = input()->findToplevel(pos)) {
+        if (Window *t = pickDragTarget(pos)) {
             // TODO: consider decorations
             if (t->surface() != seat->dragSurface()) {
                 if ((m_dragTarget = static_cast<Window *>(t->isClient() ? t : nullptr))) {
@@ -2551,6 +2548,36 @@ private:
             workspace()->takeActivity(m_dragTarget, Workspace::ActivityFlag::ActivityRaise);
         }
     }
+
+    Window *pickDragTarget(const QPointF &pos) const
+    {
+        const QList<Window *> stacking = workspace()->stackingOrder();
+        if (stacking.isEmpty()) {
+            return nullptr;
+        }
+        auto it = stacking.end();
+        do {
+            --it;
+            Window *window = (*it);
+            if (window->isDeleted()) {
+                continue;
+            }
+            if (!window->isClient()) {
+                continue;
+            }
+            if (!window->isOnCurrentActivity() || !window->isOnCurrentDesktop() || window->isMinimized() || window->isHiddenInternal()) {
+                continue;
+            }
+            if (!window->readyForPainting()) {
+                continue;
+            }
+            if (window->hitTest(pos)) {
+                return window;
+            }
+        } while (it != stacking.begin());
+        return nullptr;
+    }
+
     qint32 m_touchId = -1;
     QPointF m_lastPos = QPointF(-1, -1);
     QPointer<Window> m_dragTarget;
@@ -3158,28 +3185,12 @@ Window *InputRedirection::findToplevel(const QPointF &pos)
         return nullptr;
     }
     const bool isScreenLocked = waylandServer() && waylandServer()->isScreenLocked();
-    // TODO: check whether the unmanaged wants input events at all
     if (!isScreenLocked) {
         // if an effect overrides the cursor we don't have a window to focus
         if (effects && static_cast<EffectsHandlerImpl *>(effects)->isMouseInterception()) {
             return nullptr;
         }
-        const QList<Unmanaged *> &unmanaged = Workspace::self()->unmanagedList();
-        for (Unmanaged *u : unmanaged) {
-            if (u->hitTest(pos)) {
-                return u;
-            }
-        }
     }
-    return findManagedToplevel(pos);
-}
-
-Window *InputRedirection::findManagedToplevel(const QPointF &pos)
-{
-    if (!Workspace::self()) {
-        return nullptr;
-    }
-    const bool isScreenLocked = waylandServer() && waylandServer()->isScreenLocked();
     const QList<Window *> &stacking = Workspace::self()->stackingOrder();
     if (stacking.isEmpty()) {
         return nullptr;

src/input.h

@@ -166,7 +166,6 @@ public:
     void removeIdleInhibitor(Window *inhibitor);
 
     Window *findToplevel(const QPointF &pos);
-    Window *findManagedToplevel(const QPointF &pos);
     GlobalShortcutsManager *shortcuts() const
     {
         return m_shortcuts;