Safari :has(:empty) bug

Safari, Mac and iOS, only.

I probably got some of the details wrong, but I'm fairly certain of the gist. I commented on the relevant Bugzilla issue.
Update: Fixed.

There should not be any rule such as x:empty + y, without a :has, that holds CSS declarations and selects the same element y as in the code examples below. If there is, all styles work as intended.

If there isn't...

• Take an element x that may or may not be empty, and an element y that is styled to react to x's emptiness.
• The selector must contain a :has(:empty).
  Example selectors are y:has(x:empty)
  or z:has(x:empty) y.
• Now, on page load, x's emptiness is evaluated correctly — but only once.

However,

• IF the body vertically overflows the html element and not horizontally.
• AND element x is filled by a script that runs onload
  (onDOMContentLoaded does not trigger the bug, and neither does executing the script by a direct function call later in the HTML page.)
• THEN on page load x is treated as being empty as far as styles on y are concerned.

So far x's emptiness is only evaluated once. Whatever happens to x later, y doesn't change styles

• IF the overflow/onload bug is present
• AND y has any CSS length in vw, vh, cqw, or other viewport-dependent units
• THEN resizing the window when x is filled removes the styles from y — permanently. (On iOS, an orientationchange counts as a resize.)

If y has a display: none

• its parent must have a length in a viewport-dependent unit. In that case, resizing along the correct axis when x is filled removes the styles from y.
  (i.e. resizing horizontally when vw, cqw etc are present, or resizing vertically if vh, cvqh, etc are present)
• the requirement that body may not overflow the html element horizontally is dropped.

As far as I can see :has(x:empty) is checked for emptiness only twice:

• between DOMContentLoaded and load,
• and, only if the overflow/onload bug is present, after the first resize along the correct axis, provided a viewport-dependent length unit is present.

Most tests done on Mac 26, but iOS 18 also has this bug, and I suspect 26 has, as well.

Solving the bug

If you just want to solve the bug in your production code, try writing a selector without :has(:empty).

If you can't, add a non-:has selector such as the one below that just defines a variable you never use. It's OK to select way more elements than your :has(:empty) one does, as long as your element y is also selected.

:empty ~ * {
	--test: 'nonsense';
}

If you're OK with emptiness being evaluated only once, fire your script onDOMContentLoaded, and not onload.