[css-tables] `overflow` override needs to handle `overflow-x` and `overflow-y`

[Loirooriol]

For overflow: scroll and overflow: auto, https://www.w3.org/Style/css2-updates/REC-CSS2-20110607-errata.html#s.11.1.1b says

When used on table boxes, this value has the same meaning as 'visible'.

And equivalently from https://drafts.csswg.org/css-tables/#global-style-overrides

The overflow property on the table-root and table-wrapper box, when its value is not either visible, clip or hidden, is ignored and treated as if its value was visible.

However, overflow was a longhand on CSS2. css-overflow turned it into a shorthand, with the invariant that both overflow-x and overflow-y need to have consistent scrollability. So if one axis has a scrollable value (hidden, auto or scroll) and the other axis has a non-scrollable value (visible or clip), then the latter becomes scrollable (visible → auto, clip → hidden).

Then we need to preserve this invariant. If one axis computes to scroll or auto, then the other axis must also compute to a scrollable value. So if we make the former one behave as the non-scrollable visible, we have a scrollability mismatch. Browsers agree that then both axes must behave as visible.

[fantasai]

@Loirooriol If you have a PR for this, and all implementations agree, I'll approve it. I trust you to have checked all the relevant details. :)

[Loirooriol]

Are you thinking about a PR for css-overflow or css-tables? You are only an editor of the former... We could have this fixup in css-overflow but probably we should first change the "applies to" in #10881.