wasm: mouse up event sometimes is not generated

Occasionally emscripten does not respond with a proper mouse up event.

This seems to effect QDialog windows that are not maximized/full screen.

When I grab the window to move it around, sometimes the window does not  move as fast as the mouse. If I let go of the mouse button when it is not directly under the pointer, emscripten does not send mouse the mouse up event. This means I can move the window around just by moving the mouse.