Batch DOM checks for fallback positions

[jamesnw]

Per @mmalerba -

"I'm not very familiar with @floating-ui/dom, maybe its doing some intelligent batching already, but if not, its something we could consider doing manually. i.e. add the styles for a new candidate position for every target and then check the overflow of all of them at once, repeat the process for any that don't have a match yet"

#202 (comment)

This seems like a potential for improved performance, especially when there are multiple anchored elements with position fallback.