[css-ui] Need something to prevent modal dialogs from automatically getting user-select: none

[Loirooriol]

In #6685 (comment) we resolved that inert elements behave as user-select: none. I guess this refers to the used value.

But then https://drafts.csswg.org/css-ui-4/#valdef-user-select-auto says that user-select: auto has a used value if none if the parent has a used value of none.

This is a problem, because if you have a modal dialog, its parent (and all elements outside the dialog) becomes inert, so it has a used value of user-select: none. The dialog is not inert, so by default it gets user-select: auto, which will have a used value of none.

So not sure if user-select: auto should resolve without taking effects of inertness into account, or maybe the UA stylesheet should set user-select: all to modal dialogs (analogous to #6939), or something like that.