In vscode, tab (the key above capslock) is implemented by counting how many characters are on the current line and adding 4floor(x/4+1)-x spaces to the current line.

This obviously doesn't work if there are non-ascii characters that (have to) use a different font and hence having different widths. Or if you prefer variable-width font, then a purely ascii document will break tab.

The developers reply on github that this (characters having different widths) is a frontend thing and they don't have enough API that can fix this. But of course since vscode is an open source project you can always fork it and fix it yourself.

I don't know any other editor that cannot handle tab.