[css-anchor] Detecting active @position-fallback

[jh3y]

Currently, we don't have a proposed way to detect which fallback is currently active.

A huge use case for this is if we want to display arrows on elements such as tooltips. If I have a position-fallback that goes from top to bottom:

@position-fallback --vertical {
  @try {
    top: anchor(bottom);
  }
  @try {
    bottom: anchor(top);
  }
}

If my arrow is styled on the top of my element. But then my element is moved to be on top of the anchor. My arrow now points to space potentially.

Although not possible with the spec as is, being able to use non-inset properties would be interesting inside the fallback.

@position-fallback --vertical {
  @try {
    --on-the-bottom: 1;
    top: anchor(bottom);
  }
  @try {
    --on-the-top: 1;
    bottom: anchor(top);
  }
}

You could use that custom property like a boolean flip for the arrow position. But, I imagine there's a more elegant solution to this potentially.

[xiaochengh]

There's a fundamental issue: position-fallback is applied at used value time. If we expose the active fallback to CSS, then we allow computed values to depend on used values, which violates the "specified -> computed -> used values" pipeline.

Unless we introduce one more snapshotting: we can snapshot the active fallback at the beginning of a frame and expose it to other properties, like how we currently deal with scrolling.

[tabatkins]

The plan is that we'll introduce a new CQ type for this; CQs violate that pipeline in the exact same way, but in a controlled manner. We'll need to be careful that you can't trigger the parent to flip into a different fallback set, tho.

[bfgeek]

The plan is that we'll introduce a new CQ type for this; CQs violate that pipeline in the exact same way, but in a controlled manner. We'll need to be careful that you can't trigger the parent to flip into a different fallback set, tho.

We'll need size containment in both axes for this to work

[fantasai]

This isn't fundamentally cyclic, as long as you have a clear and unambiguous way to tie the conditional styles to the position option.

[kizu]

I think using something like CQ here is the way, though the tricky thing is the size containment.

The way I think could be the most powerful while avoiding any circularity: an ability to query any container's closest used fallback. Critically, not the fallback used on the container itself, as allowing the containment on the children of the positioned element can make things more flexible.

<div popover>
  <div class="decorations"></div>
  Some content
</div>

With this HTML, for example, we will have a popover which uses some list of fallbacks, and then what we could do is position the .decorations with inset: 0 over the popover, make it a container, and then any pseudo-element (or element) inside this container could query the fallback.

Of course, if the popover itself could be made a container, any children could query the fallback directly, but that will come with more limitations, so an ability to separate the containers from the fallbacks they query could be the most flexible way to go.