Duplicate character when long-pressing for accented char on macOS/webengine

This looks similar to https://bugreports.qt.io/browse/QTBUG-71394, but applies when using QWebEngineView. To reproduce, open the markdowneditor sample Qt app, and enter

<textarea></textarea>

on the left, so a typing box appears on the right. Focus it and hold down a, and you'll see a pop-up of available accents. Clicking on one inserts the accented character, but does not remove the unaccented character, so you end up with two characters instead of one.

Issue also exists when using a contenteditable div.