UI scrollbars clipping fix

[ickshonpe]

Objective

Fix two problems

1. Scrollbars are placed in the padding region of the node, not the content region. The content insets returned by ComputedNode::content_inset should account for the sizes of the scrollbars.
2. In update_clipping_system the clipping rect calculations are incorrect, scrollbar_size should only be added to the right and bottom when visual_area is set to ContentBox. For BorderBox and PaddingBox, the scrollbars are inside the clipping rect.

Solution

1. Add the scrollbar size to the right and bottom insets returned by ComputedNode::content_inset.
2. Since content_inset now includes scrollbar size, remove the subtraction from clip_rect.max in update_clipping_system.

Testing

Look at:

cargo run --example scrollbars --features="experimental_bevy_ui_widgets"

[kfc35]

I’m assuming this change is also relevant ui_node.rs resolve_clip_rect as well and that everything checks out there. I say this just because I see a usage of content_inset in that function.

My only small suggestion might be to note in the function comment that content_inset also includes scrollbars but probably not required?

[ickshonpe]

I’m assuming this change is also relevant ui_node.rs resolve_clip_rect as well and that everything checks out there. I say this just because I see a usage of content_inset in that function.

Yes, the code using resolve_clip_rect in focus was incorrect before and should be fixed now.

My only small suggestion might be to note in the function comment that content_inset also includes scrollbars but probably not required?

Maybe the docs should be improved somewhere, but I'd rather leave it to another PR.