In spec under Resize Observer steps, it says:

At the time that ResizeObserver events are determined and delivered:

If a box el is absolutely positioned, set its last successful position option to the set of accepted @position-try properties (and values) that it’s currently using.
Otherwise, if el has a last successful position option and if any of the following are true of it, remove its last successful position option:

• el is not absolutely positioned
• el’s computed value for position-try-fallbacks has changed
• Any of the @position-try rules referenced by el’s position-try-fallbacks have been added, removed, or mutated.

I might be parsing this wrong, but this sounds like the following:

if box el is abspos, do X
otherwise do Y if any of the following hold:

• box el is not abspos
• [...]

which is equivalent to

if box el is abspos do X, otherwise do Y

@tabatkins