Fix ScrollBar grabber mouse input ignores scroll content margins.

[WhalesState]

• Fixes: ScrollBar StyleBoxTexture Grabber only half usable? #98004

Fixes this, the grab area now will respect the left and top content margins.

Also ignoring the focus style content margins and using only the scroll style's one, to prevent the grabber from changing it's position when focused.

Before:

before.mp4

After:

after.mp4

[bruvzg]

Grab area seems to be still a bit off (red area works, the rest is still click-through):

[WhalesState]

Grab area seems to be still a bit off (red area works, the rest is still click-through):

The MRP that he has added includes expand margin for both left and right sides in the grabber, if you check it again, you will see that it's actually centered but the first and last 8 pixels can't be clicked. please test with this MRP after removing the expand margins test-scrollbar-grabber-main.zip and try to add some content margins for the scrolls.

The issue was explained in this comment #98004 (comment), The ScrollBar doesn't take the content margins into account for mouse input.