Requested feature overview description.
On the flag post page, the Inferior of Post # field should have the up and down buttons removed. This field should also be immune to scrolling.

Why would it be useful?
From a UI perspective, these buttons serve little purpose as post numbers are almost always pasted or written in, not moved towards via increment/decrement. Removing the scroll prevents the issue of someone pasting in the correct post number and then scrolling down to submit, inadvertently changing the post number. I just realized today why this issue keeps happening to me.

What part(s) of the site page(s) are affected?
The post flag page.

Makes sense

I'm pretty sure these can't be removed as a constraint of using <input type="number">.

I believe the only way to do this would be switching it to a text input and adding a filter to only allow numerical characters.

Edit:
Other things that probably need this

• Parent Post ID on the upload form
• Avatar Post ID on the user settings page
• Forum topic on new bulk update request

Browser validation looks inconsistent. Chrome seems to do it (I can still type -.e because specs), Firefox doesn't. Flags correctly throw an error when the id is non-numeric, this seems fine as is. Though adding some custom client-side validation shouldn't be hard.