rustdoc sidebar isn't wide enough

[lolbinarycat]

I believe the sidebar is currently hardcoded to 200 pixels, and the css is set to truncate long sidebar items.

I could get into a buch of edge cases like font scale, ultrawide monitors, and unusually long section names with common prefixes, but i don't need to, because you can see this problem just by looking at the standard library documentation.

I think this menu should use 90%-100% of the screen width on mobile, and we should remove the truncation so long section names can be fully read.

[lolbinarycat]

Apparently you can actually drag to resize the sidebar, however it's quite precise and the discoverability is quite bad.

[lolbinarycat]

It's not even mentioned in the rustdoc book.

And regardless, this isn't accessible to mobile users, who should still get a wider sidebar with no truncation.

[lolbinarycat]

Also, we have a weird alternative "show sidebar" icon that shows up when you resize the sidebar to 0?

[lolbinarycat]

It's also worth noting that the col-resize cursor doesn't seem to be working on my end, although changing it to e-resize did work.

[lolbinarycat]

fixed on desktop by #139562, and fixed on mobile by #139831