I was trying to recreate what seems to trigger the problem at my end, and started describing step by step as below. What I seem to see now is that so far I can't recreate it on first attempt. Maybe something has been tweaked - hmmm. Anyway, I'll leave the below as is, in case I encounter the issue again in normal use and wish to revisit this post.
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I'm not zooming, but it seems to depend how large I have the browser window. Sometimes if I have say Firefox, Outlook and Excel open then I'll have the three windows sized and arranged around the screen so I can keep an eye on all three at once. That means I tend to reduce the window sizes to focus on what I need, rather than anything excess. This feels 'SOP' to me, but perhaps it's not
Example on this window/forum - on my (about) 68cm monitor. With this window at a random 52cm the tabs on the left are fine, not blocking anything.
If I reduce the width, at 50cm those tabs are just touching (not yet obstructing) the blue background of the text area.
Reduce width further, and a few of 'auto-resizes' trigger, still fine though.
At about 40cm the tabs move, flipping to the bottom of the window! Still not obstructing anything.
[etc.... see my para 1!]