Overview
The fix for QTBUG-44313 introduced clearing QCocoaWindow::m_backingStore when QCocoaWindow::m_platformWindow->m_inSetGeometry is true. Leaving the backing store null causes a subsequent paint operation to abort, leaving the dialog momentarily undrawn (transparent).

Deeper Analysis
This line is causing the dialog to momentarily disappear from the screen:

[m_nsWindow setFrame:[m_nsWindow frameRectForContentRect:bounds] display:YES animate:NO];

That's happening because of this line deeper in the callstack:

if (m_backingStore.isNull()
        return;

Recommended Fix - DEPRECATED, SEE COMMENTS FOR REVISED FIX
I've applied this locally on top of Qt 5.4.2 with great success. Instead of clearing the backing store, do the following:

        {
            m_backingStore = QImage(geometry.size(), m_backingStore.format());
            QEvent updateRequest(QEvent::UpdateRequest);
            QCoreApplication::sendEvent(m_window, &updateRequest);
         }

That creates a new backing store at the correct new size and then forces the widget to synchronously draw at that size to fill the new backing store with correct visuals.