Preserve stacking order constraints when closing a window

When closing a window, we may need to change parent-child relationship
between windows, but we need to preserve stacking order constraints so
dialogs are placed above their parents when they're closed.

src/waylandwindow.cpp

@@ -221,16 +221,18 @@ void WaylandWindow::doSetActive()
 
 void WaylandWindow::cleanGrouping()
 {
+    // We want to break parent-child relationships, but preserve stacking
+    // order constraints at the same time for window closing animations.
+
     if (transientFor()) {
-        transientFor()->removeTransient(this);
-    }
-    for (auto it = transients().constBegin(); it != transients().constEnd();) {
-        if ((*it)->transientFor() == this) {
-            removeTransient(*it);
-            it = transients().constBegin(); // restart, just in case something more has changed with the list
-        } else {
-            ++it;
+        transientFor()->removeTransientFromList(this);
+        setTransientFor(nullptr);
     }
+
+    const auto children = transients();
+    for (Window *transient : children) {
+        removeTransientFromList(transient);
+        transient->setTransientFor(nullptr);
     }
 }
 

src/window.h

@@ -925,6 +925,7 @@ public:
     virtual QRectF transientPlacement(const QRectF &bounds) const;
     const Window *transientFor() const;
     Window *transientFor();
+    void setTransientFor(Window *transientFor);
     /**
      * @returns @c true if transient is the transient_for window for this window,
      *  or recursively the transient_for window
@@ -934,6 +935,7 @@ public:
     const QList<Window *> &transients() const; // Is not indirect
     virtual void addTransient(Window *transient);
     virtual void removeTransient(Window *transient);
+    void removeTransientFromList(Window *cl);
     virtual QList<Window *> mainWindows() const; // Call once before loop , is not indirect
     QList<Window *> allMainWindows() const; // Call once before loop , is indirect
     /**
@@ -1623,11 +1625,6 @@ protected:
     void setupWindowManagementInterface();
     void updateColorScheme();
     void ensurePalette();
-    void setTransientFor(Window *transientFor);
-    /**
-     * Just removes the @p cl from the transients without any further checks.
-     */
-    void removeTransientFromList(Window *cl);
 
     virtual Layer belongsToLayer() const;
     virtual bool belongsToDesktop() const;

src/x11window.cpp

@@ -3076,19 +3076,29 @@ void X11Window::removeFromMainClients()
 // related lists.
 void X11Window::cleanGrouping()
 {
-    removeFromMainClients();
+    // We want to break parent-child relationships, but preserve stacking
+    // order constraints at the same time for window closing animations.
+
+    if (transientFor()) {
+        transientFor()->removeTransientFromList(this);
+        setTransientFor(nullptr);
+    }
+
+    if (groupTransient()) {
+        const auto members = group()->members();
+        for (Window *member : members) {
+            member->removeTransientFromList(this);
+        }
+    }
+
+    const auto children = transients();
+    for (Window *transient : children) {
+        removeTransientFromList(transient);
+        transient->setTransientFor(nullptr);
+    }
+
     group()->removeMember(this);
     in_group = nullptr;
-
-    for (auto it = transients().constBegin(); it != transients().constEnd();) {
-        if ((*it)->transientFor() == this) {
-            removeTransient(*it);
-            it = transients().constBegin(); // restart, just in case something more has changed with the list
-        } else {
-            ++it;
-        }
-    }
-
     m_transientForId = XCB_WINDOW_NONE;
 }