[the8472]

> Most websites no longer render the offscreen content to improve performance

Which may now be obsolete. Over the last year firefox has implemented some amazing flexbox layout computation improvements that make appending to a flexbox with many elements (>5k) stay fast. It only re-computes the changed ones.

Of course you need to do some profiling to check that you don't accidentally deviate from the happy path by introducing changes that touch all elements.