Made sidebar have fixed position

[KollPotato]

Fixes #338.

Works on my machine, here is proof:

fixed.sidebar.video.mp4

[Robyt3]

How does it work on mobile?

[KollPotato]

I have tested it in the dev tools on my desktop and also on my phone, works exactly like before.

[KollPotato]

YouCut_20250823_191335036.mp4

[KollPotato]

DON'T MERGE!!!! It does not behave as expected on some pages.

[KollPotato]

I changed so if there is an overflow the sidebar is scrollable, otherwise no scrollbar is shown.

[Robyt3]

The navigation scrollbar overlapping the content doesn't look good to me. The scrollbar being visible with a common screen size like 1920x1080 also doesn't look good.

[KollPotato]

The scrollbar being visible with a common screen size like 1920x1080 also doesn't look good.

It is intentional that scrollbar only shows up when content doesn't fit screen height

[Robyt3]

This now applies a transition: 0.4s; property to all elements, so the navigation position weirdly transitions when you resize the window, and highlighting of hovered links is barely visible anymore due to the slow transition.

[KollPotato]

I got a bit lost on this problem last year xD

@Robyt3 can you send a video of the weird transition you mentioned? I am unable to notice it myself and don't think this PR could have influenced transitions of other elements.

[Robyt3]

@Robyt3 can you send a video of the weird transition you mentioned? I am unable to notice it myself and don't think this PR could have influenced transitions of other elements.

I only could only reproduce it once now in Firefox, but it doesn't seem to happen consistently and I can't get it to happen a second time. All I did was load ddnet.org with the default theme and replace the contents of css.css with the file from this PR using the browser developer console then resize the window so the navigation changes to horizontal.

Please squash the commits into one in any case.

[KollPotato]

Please squash the commits into one in any case.

Squashed and ready to go.

[def-]

Sorry for not having noticed this PR before. I don't like fixed position of sidebar, so I made it non-fixed a while ago. I like it when the entire site is one block that moves all together, as it currently does. But maybe that's just me and others think it's better?

Edit: The implementation itself seems fine, so if we want to go with it, fine to merge.