We could probably significantly avoid the amount of required reflows due to presence of mw-collapisble, by adding a styles module with something like:
.client-js .mw-collapsible.mw-collapsed .mw-collapsible-content { display: none; } .client-js table.mw-collapsible.mw-collapsed > tbody > tr:not(:first-child) { display: none; }
That won't fix them all, but a significant reduction might already be worthwhile
I made similar changes at some point for collapsible tables on en.wp and it had a significant impact on 'jumping page' complaints.
/* Reduce page jumps by hiding collapsed/dismissed content */ .client-js .collapsible.collapsed > tbody > tr:not(:first-child) { display: none; }