[css-anchor-position] An ability to have a transition for position-visibility's hidden effect

[kizu]

Right now the effect of position-visibility (https://drafts.csswg.org/css-anchor-position-1/#position-visibility) is immediate, making it sometimes a bit jarring when the anchored element hides/appears.

I can see authors wanting to design this transition: through opacity, translate or any other way they want to design the regular enter/leave transition for their anchored elements.

For popovers we can design the transition via a combination of :popover-open and @starting-style (https://nerdy.dev/using-starting-style-and-transition-behavior-for-enter-and-exit-stage-effects), but for cases like with position-visibility we don't have any hooks that we currently could use for this.

I am not yet sure what the solution could be, but I am certain that we need one. Open questions:

• Should we somehow reuse the @starting-style?
• Will we need the @ending-style or something like that in absence of :popover-open alternative?
• Or could we say that when the element hides via position-visibility it uses the @starting-style if it is defined as the styles it will transition to?
• If we'd add the above, should we have a new keyword in the transition-property to cover the visibility part of the transition itself, like position-visibility? Or we could reuse visibility there?

[kizu]

Closing as a duplicate of #10182