I did indeed try changing tab order and that made no difference. It’s the Z-Order where one control is drawn before or later than another that makes the difference.
Given that this began happening recently and that @Patrick_Salo has also experienced it under very similar conditions to mine, I have a hunch it’s similar to this thread I made about two years ago.
On windows with both DesktopHTMLViewer and WebView2ControlMBS, the order the controls are processed can cause a crash due to a Microsoft bug with the Edge/WebView2 engine. That also explains why this suddenly appeared out of nowhere: it showed up with a Microsoft Edge update.