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

In https://github.com/w3c/csswg-drafts/issues/6685#issuecomment-930305697 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.

Start with the CSS UI 4 definition of user-select: auto and the linked CSSWG issue 6685 discussion about inert elements. Compare the inherited used-value behavior with modal dialogs and review issue 6939 for the analogous proposal. Done means the specification has a decided, internally consistent treatment of text selection for modal dialogs and inert content.