recreate presentwindows grids from desktopgrid

Theory:
----------
because PresentWindowsEffect::screenCountChanged() is shortcut
for "if (!isActive())", but the desktopgrid doesn't call
PresentWindowsEffect::setActive (or at least
PresentWindowsEffect::screenCountChanged), so the effect can
"miss" the increasing screen count change (it sees the signal,
but ignores it) and when desktopgrid calls it, it assumes the
m_gridSizes array is big enough (but it isn't)

Steps:
----------
1. effects are loaded, 1 screen present
2. 2nd screen gets added, but inactive effects ignore that
3. desktop grid gets activated, updates according to screen count,
   calls presentwindows for screen #2
4. presentwindows data is only prepared for one screen from step 1
   => BOOM

BUG: 351724
CCBUG: 326032
FIXED-IN: 5.4.2
REVIEW: 124960

effects/desktopgrid/desktopgrid.cpp

@@ -1098,6 +1098,7 @@ void DesktopGridEffect::setup()
     if (m_usePresentWindows)
         m_proxy = static_cast<PresentWindowsEffectProxy*>(effects->getProxy(BuiltInEffects::nameForEffect(BuiltInEffect::PresentWindows)));
     if (isUsingPresentWindows()) {
+        m_proxy->reCreateGrids(); // revalidation on multiscreen, bug #351724
         for (int i = 1; i <= effects->numberOfDesktops(); i++) {
             for (int j = 0; j < effects->numScreens(); j++) {
                 WindowMotionManager manager;

effects/presentwindows/presentwindows.cpp

@@ -96,7 +96,12 @@ PresentWindowsEffect::PresentWindowsEffect()
     connect(effects, SIGNAL(windowDeleted(KWin::EffectWindow*)), this, SLOT(slotWindowDeleted(KWin::EffectWindow*)));
     connect(effects, SIGNAL(windowGeometryShapeChanged(KWin::EffectWindow*,QRect)), this, SLOT(slotWindowGeometryShapeChanged(KWin::EffectWindow*,QRect)));
     connect(effects, SIGNAL(propertyNotify(KWin::EffectWindow*,long)), this, SLOT(slotPropertyNotify(KWin::EffectWindow*,long)));
-    connect(effects, &EffectsHandler::numberScreensChanged, this, &PresentWindowsEffect::screenCountChanged);
+    connect(effects, &EffectsHandler::numberScreensChanged, this,
+        [this] {
+            if (isActive())
+                reCreateGrids();
+        }
+    );
 }
 
 PresentWindowsEffect::~PresentWindowsEffect()
@@ -1489,7 +1494,7 @@ void PresentWindowsEffect::setActive(bool active)
         m_hasKeyboardGrab = effects->grabKeyboard(this);
         effects->setActiveFullScreenEffect(this);
 
-        screenCountChanged();
+        reCreateGrids();
 
         rearrangeWindows();
         setHighlightedWindow(effects->activeWindow());
@@ -1866,10 +1871,8 @@ bool PresentWindowsEffect::isActive() const
     return m_activated || m_motionManager.managingWindows();
 }
 
-void PresentWindowsEffect::screenCountChanged()
+void PresentWindowsEffect::reCreateGrids()
 {
-    if (!isActive())
-        return;
     m_gridSizes.clear();
     for (int i = 0; i < effects->numScreens(); ++i) {
         m_gridSizes.append(GridSize());

effects/presentwindows/presentwindows.h

@@ -225,11 +225,11 @@ public Q_SLOTS:
 private Q_SLOTS:
     void closeWindow();
     void elevateCloseWindow();
-    void screenCountChanged();
 
 protected:
     // Window rearranging
     void rearrangeWindows();
+    void reCreateGrids();
     void calculateWindowTransformations(EffectWindowList windowlist, int screen,
                                         WindowMotionManager& motionManager, bool external = false);
     void calculateWindowTransformationsClosest(EffectWindowList windowlist, int screen,

effects/presentwindows/presentwindows_proxy.cpp

@@ -39,4 +39,9 @@ void PresentWindowsEffectProxy::calculateWindowTransformations(EffectWindowList
     return m_effect->calculateWindowTransformations(windows, screen, manager, true);
 }
 
+void PresentWindowsEffectProxy::reCreateGrids()
+{
+    m_effect->reCreateGrids();
+}
+
 } // namespace

effects/presentwindows/presentwindows_proxy.h

@@ -35,6 +35,8 @@ public:
 
     void calculateWindowTransformations(EffectWindowList windows, int screen, WindowMotionManager& manager);
 
+    void reCreateGrids();
+
 private:
     PresentWindowsEffect* m_effect;
 };