This is the same bug that I reported here: https://forum.ui.vision/t/in-firefox-attempting-to-adjust-ui-vision-rpa-window-causes-all-tabs-and-buttons-to-disappear/4294
However, this time I have more information.
To adjust the vertical separation bar (the bar), you grab with the mouse and move the bar right or left and release.
IF you release inside the window, you see the problem. The bar jumps much wider than where you release. HOWEVER, if you release outside the window, such as release over the desktop or over another window, then the adjustment works perfectly.
I hope this post provides a clue to the problem. At least this post provides a workaround: Make sure to release the bar with the mouse outside the window.