(Obviously, this example requires firefox and (optionally) chromium-browser.
Javascript must be enabled as well)
Put this html snippet into a file named e.g. popup_test.html,
then open in firefox browser, or invoke from the commandline via:
% firefox popup_test.html
When you click the button above, an empty,
resizable popup window will be presented.
Try resizing from each corner, and from each edge.
The failure is this: all four corners of the popup window will move during resizing.
Expected result: at most three corners should move (when dragging a corner of the window)
or at most two (when dragging an edge of the window).
(This demo works (i.e. reproduces the failure) in firefox on latest Ubuntu 20.04)
Meanwhile, the popup works fine if this example is opened in chromius-browser
(or invoked from the commandline like this):
% chromium-browser popup_test.html