Ctrl+Backspace and other keyboard shortcuts do not work on Mac with Google Japanese IME

Steps to reproduce:
1. Select the Alphanumeric mode in Google Japanese IME on a Mac.
2. Assign the key combination Ctrl+Backspace to an editor function, for example DeleteStartOfWordCamelCase.
3. Press ctrl-backspace with the cursor in some text.

Expected result: delete backwards by camelCase.
Actual result: nothing happens.

This has been a problem for me for at least 1 year probably 2+.

I rebuilt Creator and did some experiments. When the shortcut is not set, the QKeyEvent passed to TextEditorWidget::keyPressEvent() seems identical using both Google IME and the Mac default 'kotoeri', as far as I can see using the debugger.

I believe that the ctrl-backspace event gets to QWindowSystemInterface::handleShortcutEvent() but does not continue to QShortcutMap::tryShortcut(). Looking at the source I would guess that an event filter somewhere is accepting the QEvent::ShortcutOverride, but I am not sure.

This issue is possible related to QTBUG-57934 and/or QTBUG-53804.