>Are you using 125% resolution as I do?

OK, bingo, there you go. You have found the reason for that. Either in regular mode or Tab mode, this is flickering but not always. If I resize slowly the browser window, I can see the normal behavior and, with only one pixel smaller, I can see the switch to the browser scrollbars. The test has been done at 125%. I assume this is a bug. Because, if you do the same simulation as I did, you should be able to see that when resizing slowly, by diminishing the browser window, that those unwanted scrollbars go on and off.

Can you let me know that at 100% you do not have this behavior?

Can you also let me know that the above simulation is acting also weird in your browser?