This is kinda related to one issue that got fixed with the tabs a long time ago, from the thread here:
https://forums.opera.com/topic/53115/tab-strip-change-size-when-i-close-a-tab?_=1651912581671
However, it hasn't been fully fixed, there's still an issue with the tabs changing size when you change to a different one, while the tabs are still "compressed", even though you didn't leave the tabs area.
An example, showing two versions, the last version that worked "properly" and then one of the next versions where the issue is:
https://streamable.com/1wjvki
(it's not the latest version because I recorded it a while back, but I checked it with the latest opera 88.0.4401.0 developer build and it's still there)