Using both 5.15.0 and 5.15.2, with emscripten 1.39.8 and 1.39.15.  When compiled as x86_64 native binary app, qml MessageDialog opens in a separate window correctly.

When run as webassembly, any popups fail to open new window and consume the space of the parent.  Clicking a button to close results in a refresh failure and controls remain on screen until the browser window is resized.  Then, the screen underneath is repainted.

This happens using chrome, chromium, firefox, edge, etc.