[css-ui-4] add the window-drag text

[kyerebo]

This change builds off of @diekus' PR to add the window-drag (formerly app-region) CSS property to the css-ui-4 spec. It makes requested edits from the previous PR, as well as adds a note mentioning app-region as an alias as a result of the new resolution at #13102.

This comes out of resolutions #13102 and
#13101 taken on 21/01/26.

[kyerebo]

Hi @emilio! I noticed you were reviewing Diego's original PR- I've applied the edits you had asked for there. Do you mind taking a look at this one as well?

[emilio]

Does this property have an effect on event dispatch? I'm assuming that elements with window-drag: move will not get "web" drag events.

The spec should say what the expectation is for something like <div draggable=true style="window-drag:move">.

Looks good to me with these things documented and the questions below asked.

[kyerebo]

I've added a new text blurb that clarifies event dispatch behavior, as well as clarifying draggable behavior. Please let me know if the changes look good to you!

[emilio]

Looks reasonable to me, but an editor of this spec should also review it :)

[kyerebo]

@tabatkins @frivoal Hi Editors! Could I ask one of you to take a look at this spec text for window-drag? Happy to make any changes, thank you for your time.

[kyerebo]

@fantasai Noticed you put the following in the older PR:

This probably needs a bit more clarification on what part of the element can be used as a drag target (by tying it to some other definitions, I'm not sure which ones).

Do you think specifying something like-

Specifies that the element's border box can be used as a drag area.

-would be clear enough?

[fantasai]

@kyerebo Technically it's the hit-testing area, whatever that is. I don't know if we define this concept anywhere though? Maybe just use [=principal box=] for now. I think that'll probably get the point across.

[kyerebo]

Clarified [=principal box=] as the specific drag target for the element, thanks!

[kyerebo]

Hi @tabatkins @frivoal ! I think the change is in a pretty solid state now with reviews, do you think one of you could take a final look, and help me with getting it merged?

[tabatkins]

Besides my one question, LGTM

[kyerebo]

@tabatkins Thank you for the review and writing changes!
If my response to the question is satisfactory, do you think you could leave an approval and help me with merging it in? (I don't have permissions unfortunately.)
Otherwise, I'm happy to make any other changes or continue to discuss.