Open a lot of tabs.
1) Close the last tab: all the other tabs will increase their widths and thus the space occupied by the tabs will remain the same as before.
2) Close another tab (but not the last): the tabs won't increase their widths and so the other tabs on the right will move to the left.
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Well on my observation is that 2) only remain same width if the mouse are there, after the mouse move away the tab width auto increase their width like 1).
This kind of tabs behavior also present in chrome and firefox as well.
That's weird. I installed Edge Canary now on another computer and it worked exactly as you described it.
But on this computer I'm using right now this isn't happening.
Well, that means there must be something wrong with my computer and not with Edge Chromium.
I will uninstall and install again to see if this strange behavior disappears.