Return to the two-step destruction in compositor

The two-step destruction we used to employ is needed as destroying
a screen which contains a window crashes.

Change-Id: I722828be5408a7f079d66e845eeee34ed19cbf34
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>
(cherry picked from commit 644036b597137306959d875127cc819612ee9217)
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/plugins/platforms/wasm/qwasmcompositor.cpp

@@ -65,10 +65,14 @@ QWasmCompositor::~QWasmCompositor()
     if (m_requestAnimationFrameId != -1)
         emscripten_cancel_animation_frame(m_requestAnimationFrameId);
 
-    deregisterEventHandlers();
     destroy();
 }
 
+void QWasmCompositor::onScreenDeleting()
+{
+    deregisterEventHandlers();
+}
+
 void QWasmCompositor::deregisterEventHandlers()
 {
     QByteArray screenElementSelector = screen()->eventTargetId().toUtf8();

src/plugins/platforms/wasm/qwasmcompositor.h

@@ -50,6 +50,8 @@ public:
     void raise(QWasmWindow *window);
     void lower(QWasmWindow *window);
 
+    void onScreenDeleting();
+
     QWindow *windowAt(QPoint globalPoint, int padding = 0) const;
     QWindow *keyWindow() const;
 

src/plugins/platforms/wasm/qwasmscreen.cpp

@@ -92,8 +92,6 @@ QWasmScreen::QWasmScreen(const emscripten::val &containerOrCanvas)
 
 QWasmScreen::~QWasmScreen()
 {
-    Q_ASSERT(!m_compositor); // deleteScreen should have been called to remove this screen
-
     emscripten::val::module_property("specialHTMLTargets")
             .set(eventTargetId().toStdString(), emscripten::val::undefined());
 
@@ -103,9 +101,7 @@ QWasmScreen::~QWasmScreen()
 
 void QWasmScreen::deleteScreen()
 {
-    // Delete the compositor before removing the screen, since its destruction routine needs to use
-    // the fully operational screen.
-    m_compositor.reset();
+    m_compositor->onScreenDeleting();
     // Deletes |this|!
     QWindowSystemInterface::handleScreenRemoved(this);
 }