[Question] Support for :top-layer content, e.g. <dialog> and [popup]

Content in a <dialog> that is opened with showModal() or showPopUp() via the popup attribute will not actually have “offsetParents” or “clippingAncestors”. How do you think it is best for the library to support positioning that content?

Particularly, if a reference element and a floating element are both bound in an element with a matrix transform, a la https://codesandbox.io/s/floating-ui-dom-template-forked-es3dfl?file=/src/index.js, do you foresee adding extra checks for this reality, or surfacing a flag so a use can opt-out of ancestor math or something else.

[Westbrook]

I’m testing out this fairly naive middleware, based on the offset middleware, to manage this context:

import { Middleware, MiddlewareArguments } from "@floating-ui/dom";

export const topLayer = (): Middleware => ({
  name: 'topLayer',
  async fn(middlewareArguments: MiddlewareArguments) {
    const { x, y, elements: { reference, floating } } = middlewareArguments;
    let onTopLayer = false;
    const diffCoords = {
        x: 0,
        y: 0,
    }
    // browsers will throw when they do not support the following selectors, catch the errors.
    try {
        onTopLayer = onTopLayer || floating.matches(':open');
    // eslint-disable-next-line no-empty
    } catch (e) {}
    try {
        onTopLayer = onTopLayer || floating.matches(':modal');
    // eslint-disable-next-line no-empty
    } catch (e) {}

    if (onTopLayer) {
        // :top-layer content will be positioned from `window` get the reference's position from there, too.
        const rect = reference.getBoundingClientRect()
        diffCoords.x = rect.x;
        diffCoords.y = rect.y;
    }

    return {
      x: x + diffCoords.x,
      y: y + diffCoords.y,
      data: diffCoords,
    };
  },
});

This feels like a situation that will come up more and more as these specs solidify and expect. It feels like this should be core library functionality, or at least a core library middleware.

[Westbrook]

Can do! My team and I are specifically looking into all of this right now, so hopefully I can get your something this week.