Change the default visual box for OverflowClipMargin to PaddingBox

[ickshonpe]

Objective

The default should be OverflowClipBox::PaddingBox not OverflowClipBox::ContentBox

padding-box is the default in CSS.

Solution

Set the default to PaddingBox.

Testing

Compare the overflow UI example on main vs with this PR. You should see that on main the outline around the inner node gets clipped. With this PR by default clipping starts at the inner edge of the border (the padding-box) and the outlines are visible.

Fixes #18934

[github-actions]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Please review the instructions for writing migration guides, then expand or revise the content in the migration guides directory to reflect your changes.

[Henauxg]

Found a link to a CSS spec for the default value: https://udn.realityripple.com/docs/Mozilla/Gecko/Chrome/CSS/overflow-clip-box
(Btw, it seems that overflow-clip-box is an obsolete CSS property, not supported in modern browsers)

UI overflow example

On main:

On this PR:

[NthTensor]

Migration guide looks good. Thanks!

[ickshonpe]

Found a link to a CSS spec for the default value: https://udn.realityripple.com/docs/Mozilla/Gecko/Chrome/CSS/overflow-clip-box (Btw, it seems that overflow-clip-box is an obsolete CSS property, not supported in modern browsers)

Yeah I think the visual-box settings are all just rolled into clip-path in modern CSS. We could call rename the enum to ClipPath but it seems a bit deceptive since we don't support arbitrary clipping shapes atm, only rectangles.