Fix crash when extending selection after switching between primary and secondary screen

BUG: 398320

src/ScreenWindow.cpp

@@ -49,6 +49,11 @@ void ScreenWindow::setScreen(Screen *screen)
 {
     Q_ASSERT(screen);
 
+    if (screen == _screen) {
+        return;
+    }
+
+    emit screenAboutToChange();
     _screen = screen;
 }
 

src/ScreenWindow.h

@@ -257,6 +257,11 @@ Q_SIGNALS:
     /** Emitted when the selection is changed. */
     void selectionChanged();
 
+    /** Emitted when e. g. Screen is changed (to alternate or primary), so
+     * initial selection offsets etc. are invalidated.
+     */
+    void screenAboutToChange();
+
 private:
     Q_DISABLE_COPY(ScreenWindow)
 

src/widgets/TerminalDisplay.cpp

@@ -139,6 +139,11 @@ void TerminalDisplay::setScreenWindow(ScreenWindow* window)
         connect(_screenWindow.data(), &Konsole::ScreenWindow::outputChanged, this, [this]() {
             _filterUpdateRequired = true;
         });
+        connect(_screenWindow.data(), &Konsole::ScreenWindow::screenAboutToChange, this, [this]() {
+            _iPntSel = QPoint();
+            _pntSel = QPoint();
+            _tripleSelBegin = QPoint();
+        });
         connect(_screenWindow.data(), &Konsole::ScreenWindow::scrolled, this, [this]() {
             _filterUpdateRequired = true;
         });