Prevent layout shift in <overflow-menu> items

[silverwind]

There is a small layout shift in when active tab changes. Notice how the actions SVG is unstable:

This is because the active item with bold text is wider then the inactive one. I have applied this trick to prevent this layout shift. It's only active inside <overflow-menu> because I wanted to avoid changing HTML and doing it in regular JS would cause a flicker. I don't expect us to introduce other similar menus without <overflow-menu>, so that place is likely fine.

I also changed the weight from 500 to 600, slightly reduced horizontal padding, merged some tab-bar related CSS rules and a added a small margin below repo-header so it does not look so crammed against the buttons on top.

[wxiaoguang]

I would prefer either:

• Make all "tab" items use the same normal font weight
• Make all "tab" items use the same semi-bold font weight

Instead of this trick .....

TBH I do not see toggling the font weight for tab items would really help the user experence.

[silverwind]

I do prefer active tab to be bold, it gives a visual distinction. Imho, it's better than color alone. The only other option i consider is making all normal weight and distinguish by color alone.