Dialog handling lost upon dragging

Qt 5.15.8 and above has dialog moving/mouse event synchonisation issues when using on WebAssembly platform.

Ex. https://nagyimre.com/testwasm/testwasm.html

Select Threads > Start, a dialog should appear. Try to move the dialog around. Grab at the bar and move around. If you do it slowly, it seems that the dialog follows the mouse movement, but if you try to move the dialog faster, the mouse pointer gets away from the dialog and the dialog is just "lost". 

Same happens with with larger scrollareas: when scrollbar handler is dragged, most of the time the scrollbar handle does not follow the mouse movement, the mouse cursor runs out from it and the scrolling stops.