Make ScrollBar account for its margins

[atlas-wheatear]

This is a bug fix for #79994 .

There are two main changes:

1. adjust the various components of the scrollbar and its background, so that they take into account the margins.
2. remove the redundant method get_area_offset

Fixed view:

new_after.mov

The old version, off of the current master branch:

old_master.mov

• Bugsquad edit, fixes: Editor scroll bar's gutter don't reach its ends #79994

[AThousandShips]

Can you provide a capture of before and after to make it easier to confirm the fix?

[atlas-wheatear]

Apologies, it's not very clear!

If you know how to change the colour of the "scroll-bar background", for ease of visual discernment, that would be much appreciated!

[AThousandShips]

Can you please squash your commits, per our guidelines

[AThousandShips]

Please name your commit something more descriptive, the title of this PR is good, use git commit --amend

[atlas-wheatear]

Lots of git push -f noise, sorry 😅 I accidentally added a merge commit to the source branch, on my fork, and had to try again.

[kleonc]

cc @YuriSizov #79994 (comment)

[KoBeWi]

This fix isn't really correct. Content margin should not affect drawing, it's there to define padding for the child content (e.g. child nodes of a PanelContainer or anything that draws over stylebox). In that regard the current behavior is correct and the fix should be done in the theme.

With your changes, the stylebox is smaller when it has margins:

vs new:

Also, since you only changed the drawing code, it doesn't correctly detect hover:

godot.windows.editor.dev.x86_64_61V8dkNfe0.mp4

[atlas-wheatear]

I don't have the free time to look at this any longer. There's an awful lot of interacting pieces!

Hopefully the gathered information (breaking commit version, etc) is useful to somebody else.

[YuriSizov]

Thanks for your efforts anyway!