[effects/cubeslide] Cancel active animation when number of desktops has changed

Summary:
If a virtual desktop is removed, then desktopChanged will be followed by
numberDesktopsChanged signal. In which case, we have to cancel the
active animation because front_desktop might be no longer valid when
it's time to perform compositing.

BUG: 406452

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D21064

effects/cubeslide/cubeslide.cpp

@@ -49,6 +49,8 @@ CubeSlideEffect::CubeSlideEffect()
             this, &CubeSlideEffect::slotWindowStepUserMovedResized);
     connect(effects, &EffectsHandler::windowFinishUserMovedResized,
             this, &CubeSlideEffect::slotWindowFinishUserMovedResized);
+    connect(effects, &EffectsHandler::numberDesktopsChanged,
+            this, &CubeSlideEffect::slotNumberDesktopsChanged);
     reconfigure(ReconfigureAll);
 }
 
@@ -644,4 +646,26 @@ bool CubeSlideEffect::isActive() const
     return !slideRotations.isEmpty();
 }
 
+void CubeSlideEffect::slotNumberDesktopsChanged()
+{
+    // This effect animates only aftermaths of desktop switching. There is no any
+    // way to reference removed desktops for animation purposes. So our the best
+    // shot is just to do nothing. It doesn't look nice and we probaby have to
+    // find more proper way to handle this case.
+
+    if (!isActive()) {
+        return;
+    }
+
+    for (EffectWindow *w : staticWindows) {
+        w->setData(WindowForceBlurRole, QVariant());
+        w->setData(WindowForceBackgroundContrastRole, QVariant());
+    }
+
+    slideRotations.clear();
+    staticWindows.clear();
+
+    effects->setActiveFullScreenEffect(nullptr);
+}
+
 } // namespace

effects/cubeslide/cubeslide.h

@@ -78,6 +78,7 @@ private Q_SLOTS:
     void slotDesktopChanged(int old, int current, EffectWindow* w);
     void slotWindowStepUserMovedResized(KWin::EffectWindow *w);
     void slotWindowFinishUserMovedResized(KWin::EffectWindow *w);
+    void slotNumberDesktopsChanged();
 
 private:
     enum RotationDirection {