position: sticky works on some mobile browsers now, so you can make a menu bar scroll with the page but then stick to the top of the viewport whenever the user scrolls past it.
But what if you want to restyle your sticky menu bar slightly whenever it's currently 'sticking'? eg, you might want the bar to have rounded corners whenever it's scrolling with the page, but then as soon as it sticks to the top of the viewport, you want to get rid of the top rounded corners, and add a little drop shadow underneath it.
Is there any kind of pseudoselector (eg ::stuck) to target elements that have position: sticky and are currently sticking? Or do browser vendors have anything like this in the pipeline? If not, where would I request it?
NB. javascript solutions are not good for this because on mobile you usually only get a single scroll event when the user releases their finger, so JS can't know the exact moment that the scroll threshold was passed.
There is currently no selector that is being proposed for elements that are currently 'stuck'. The Postioned Layout module where position: sticky is defined does not mention any such selector either.
Feature requests for CSS can be posted to the www-style mailing list. I believe a :stuck pseudo-class makes more sense than a ::stuck pseudo-element, since you're looking to target the elements themselves while they are in that state. In fact, a :stuck pseudo-class was discussed some time ago; the main complication, it was found, is one that plagues just about any proposed selector that attempts to match based on a rendered or computed style: circular dependencies.
In the case of a :stuck pseudo-class, the simplest case of circularity would occur with the following CSS:
:stuck { position: static; /* Or anything other than sticky/fixed */ }
:not(:stuck) { position: sticky; /* Or fixed */ }
And there could be many more edge cases that would be difficult to address.
While it's generally agreed upon that having selectors that match based on certain layout states would be nice, unfortunately major limitations exist that make these non-trivial to implement. I wouldn't hold my breath for a pure CSS solution to this problem anytime soon.