TabBox: Allow switchers to handle the tabbox hiding

Previously, the switcher item got hidden immediately after
selecting a window, so it wasn't possible to show an exit
transition.

Emit instead an `aboutToHide` signal and let the switchers
that opt-in to handle when to hide the tabbox by just setting
its `visible` property to `false`. In the default case we handle
that signal by immediately hiding the tabbox as usual.

For symmetry, add also a new `aboutToShow` signal which
simplifies setting an enter transition.

src/kcms/tabbox/layoutpreview.h

@@ -114,6 +114,9 @@ Q_SIGNALS:
     void screenGeometryChanged();
     void itemChanged();
 
+    void aboutToShow();
+    void aboutToHide();
+
 private:
     QAbstractItemModel *m_model;
     QObject *m_item;

src/tabbox/switcheritem.cpp

@@ -27,6 +27,7 @@ SwitcherItem::SwitcherItem(QObject *parent)
     , m_visible(false)
     , m_allDesktops(false)
     , m_currentIndex(0)
+    , m_automaticallyHide(false)
 {
     m_selectedIndexConnection = connect(tabBox, &TabBoxHandler::selectedIndexChanged, this, [this] {
         if (isVisible()) {
@@ -104,6 +105,20 @@ void SwitcherItem::setNoModifierGrab(bool set)
     Q_EMIT noModifierGrabChanged();
 }
 
+bool SwitcherItem::automaticallyHide() const
+{
+    return m_automaticallyHide;
+}
+
+void SwitcherItem::setAutomaticallyHide(bool value)
+{
+    if (m_automaticallyHide == value) {
+        return;
+    }
+    m_automaticallyHide = value;
+    Q_EMIT automaticallyHideChanged();
+}
+
 bool SwitcherItem::compositing()
 {
     return Compositor::compositing();

src/tabbox/switcheritem.h

@@ -23,11 +23,12 @@ class SwitcherItem : public QObject
     Q_OBJECT
     Q_PROPERTY(QAbstractItemModel *model READ model NOTIFY modelChanged)
     Q_PROPERTY(QRect screenGeometry READ screenGeometry NOTIFY screenGeometryChanged)
-    Q_PROPERTY(bool visible READ isVisible NOTIFY visibleChanged)
+    Q_PROPERTY(bool visible READ isVisible WRITE setVisible NOTIFY visibleChanged)
     Q_PROPERTY(bool allDesktops READ isAllDesktops NOTIFY allDesktopsChanged)
     Q_PROPERTY(int currentIndex READ currentIndex WRITE setCurrentIndex NOTIFY currentIndexChanged)
     Q_PROPERTY(bool noModifierGrab READ noModifierGrab NOTIFY noModifierGrabChanged)
     Q_PROPERTY(bool compositing READ compositing NOTIFY compositingChanged)
+    Q_PROPERTY(bool automaticallyHide READ automaticallyHide WRITE setAutomaticallyHide NOTIFY automaticallyHideChanged)
 
     /**
      * The main QML item that will be displayed in the Dialog
@@ -52,12 +53,14 @@ public:
         return m_noModifierGrab;
     }
     bool compositing();
+    bool automaticallyHide() const;
 
     // for usage from outside
     void setModel(QAbstractItemModel *model);
     void setAllDesktops(bool all);
     void setVisible(bool visible);
     void setNoModifierGrab(bool set);
+    void setAutomaticallyHide(bool value);
 
 Q_SIGNALS:
     void visibleChanged();
@@ -68,6 +71,10 @@ Q_SIGNALS:
     void itemChanged();
     void noModifierGrabChanged();
     void compositingChanged();
+    void automaticallyHideChanged();
+
+    void aboutToShow();
+    void aboutToHide();
 
 private:
     QAbstractItemModel *m_model;
@@ -77,6 +84,7 @@ private:
     int m_currentIndex;
     QMetaObject::Connection m_selectedIndexConnection;
     bool m_noModifierGrab = false;
+    bool m_automaticallyHide = true;
 };
 
 inline QAbstractItemModel *SwitcherItem::model() const

src/tabbox/tabboxhandler.cpp

@@ -316,6 +316,19 @@ void TabBoxHandlerPrivate::show()
         item->setAllDesktops(config.clientDesktopMode() == TabBoxConfig::AllDesktopsClients);
         item->setCurrentIndex(indexRow);
         item->setNoModifierGrab(q->noModifierGrab());
+        Q_EMIT item->aboutToShow();
+
+        // When SwitcherItem gets hidden, destroy also the window and main item
+        QObject::connect(item, &SwitcherItem::visibleChanged, q, [this, item]() {
+            if (!item->isVisible()) {
+                if (QQuickWindow *w = window()) {
+                    w->hide();
+                    w->destroy();
+                }
+                m_mainItem = nullptr;
+            }
+        });
+
         // everything is prepared, so let's make the whole thing visible
         item->setVisible(true);
     }
@@ -391,14 +404,12 @@ void TabBoxHandler::hide(bool abort)
     }
 #ifndef KWIN_UNIT_TEST
     if (SwitcherItem *item = d->switcherItem()) {
+        Q_EMIT item->aboutToHide();
+        if (item->automaticallyHide()) {
             item->setVisible(false);
         }
-#endif
-    if (QQuickWindow *w = d->window()) {
-        w->hide();
-        w->destroy();
     }
-    d->m_mainItem = nullptr;
+#endif
 }
 
 QModelIndex TabBoxHandler::nextPrev(bool forward) const