tiling: Evacuate tiled windows from custom & quick tiling on output removal

Context: If a display is removed, the corresponding TileManager is removed with it. This in turn removes every one of its Tiles with it, and when a Tile's destructor is called, it attempts to find a new replacement tile for any windows it was previously managing.

However, if the Tile is removed because its corresponding TileManager has been removed, this has the potential to cause a segfault in KWin, causing it to crash (I suspect a possible race condition? but not sure).

This MR correctly evacuates custom tiled windows & migrates quick tiled windows upon output removal.

BUG: 465522

src/tiles/tile.cpp

@@ -372,6 +372,14 @@ Tile *Tile::parentTile() const
     return m_parentTile;
 }
 
+void Tile::visitDescendants(std::function<void(const Tile *child)> callback) const
+{
+    callback(this);
+    for (const Tile *child : m_children) {
+        child->visitDescendants(callback);
+    }
+}
+
 TileManager *Tile::manager() const
 {
     return m_tiling;

src/tiles/tile.h

@@ -98,6 +98,11 @@ public:
      */
     QList<Tile *> descendants() const;
 
+    /**
+     * Visit all tiles descendant of this tile, recursive
+     */
+    void visitDescendants(std::function<void(const Tile *child)> callback) const;
+
     void resizeFromGravity(Gravity gravity, int x_root, int y_root);
 
     Q_INVOKABLE void resizeByPixels(qreal delta, Qt::Edge edge);

src/workspace.cpp

@@ -1595,12 +1595,51 @@ void Workspace::updateOutputs(const QVector<Output *> &outputOrder)
         Q_EMIT outputAdded(output);
     }
 
-    desktopResized();
-
     const auto removed = oldOutputsSet - outputsSet;
     for (Output *output : removed) {
-        m_tileManagers.erase(output);
         Q_EMIT outputRemoved(output);
+        auto tileManager = std::move(m_tileManagers[output]);
+        m_tileManagers.erase(output);
+
+        // Evacuate windows from the defunct custom tile tree.
+        tileManager->rootTile()->visitDescendants([](const Tile *child) {
+            const QList<Window *> windows = child->windows();
+            for (Window *window : windows) {
+                window->setTile(nullptr);
+            }
+        });
+
+        // Migrate windows from the defunct quick tile to a quick tile tree on another output.
+        static constexpr QuickTileMode quickTileModes[] = {
+            QuickTileFlag::Left,
+            QuickTileFlag::Right,
+            QuickTileFlag::Top,
+            QuickTileFlag::Bottom,
+            QuickTileFlag::Top | QuickTileFlag::Left,
+            QuickTileFlag::Top | QuickTileFlag::Right,
+            QuickTileFlag::Bottom | QuickTileFlag::Left,
+            QuickTileFlag::Bottom | QuickTileFlag::Right,
+        };
+
+        for (const QuickTileMode &quickTileMode : quickTileModes) {
+            Tile *quickTile = tileManager->quickTile(quickTileMode);
+            const QList<Window *> windows = quickTile->windows();
+            if (windows.isEmpty()) {
+                continue;
+            }
+
+            Output *bestOutput = outputAt(output->geometry().center());
+            Tile *bestTile = m_tileManagers[bestOutput]->quickTile(quickTileMode);
+
+            for (Window *window : windows) {
+                window->setTile(bestTile);
+            }
+        }
+    }
+
+    desktopResized();
+
+    for (Output *output : removed) {
         output->unref();
     }