Created attachment 390884 [details] Testcase Loading at the testcase on iOS causes the page to scroll all the way to the left, but it should remain scrolled to the right. There are some ordering issues where we fail to compute the non-zero initial snap index in RTL.
The problem is that we push data into the scrolling tree via RenderLayerCompositor::updateScrollingNodeForScrollingRole(), but this runs before updateScrollSnapState() which runs as a post-layout task.
<rdar://problem/69443111>
I think this is working now? Perhaps this was improved by recent fixes around treating points as scroll offsets versus scroll positions in ScrollableArea?
I still see issues when first loading the test case on iPhone, with iOS 15.