On the WebAssembly Platform, Qt creates a div with contenteditable="true" around the canvas it paints the window in.

Unfortunately it is possible to navigate to that div by repeatedly pressing tab, and entering text outside of the Qt window.

The result looks like this:

Changing contenteditable to false in the devtools removes the problem.

The relevant code seems to be this: https://github.com/qt/qtbase/blob/30fd101b558112b8829d3ae63b58e8504bbe9297/src/plugins/platforms/wasm/qwasmwindow.cpp#L9 (GitHub for easier linking to the relevant line)