The “New Topic/Post” boxes seem to not fill the whole width of the browser window when it is sufficiently wide. This has the strange effect of making the box larger when I resize my browser window to be smaller, which seems very odd to me. See the below screen recording:
Screen Recording

I do think that it makes sense to not fill the whole width if it’s too large, but It makes it kind of awkward on my display size. Maybe there could be an option to toggle this feature on or off?
I think it could also help if the box was more aligned with the rest of the elements of the page (i.e., only covering the topic list on large displays, not part of the sidebar), or actually be centered in the middle of the page instead of being oddly off-center.
(Of course, these ideas are just suggestions.)