Replace per-frame heap-allocated RefCounted ScrollFrame objects with a
single contiguous Vector<ScrollFrame> inside ScrollState. All frames for
a viewport are now stored in one allocation, using type-safe
ScrollFrameIndex instead of RefPtr pointers.
This reduces allocation churn, improves cache locality, and moves
parent-chain traversal (cumulative offset, nearest scrolling ancestor)
into ScrollState — similar to how visual context nodes were recently
consolidated into AccumulatedVisualContextTree.
This was arguably put in a worse place by #8162; we mostly need the
device pixel offsets from the scroll state so keep track of those and
convert back to CSS pixels when necessary (i.e. scrollbar data).
Stop converting between CSS and device pixels as part of rendering - the
display list should be as simple as possible, so convert to DevicePixels
once when constructing the display list.
Remove the public own_offset() getter from ScrollFrame and the
own_offset_for_frame_with_id() helper from ScrollState. The snapshot
creation in ScrollStateSnapshot now accesses m_own_offset directly
through a friend declaration. This ensures scroll offsets in the
painting pipeline are only read through snapshots.
The cumulative_offset was being tracked in ScrollStateSnapshot and
ScrollState but was never actually used. This simplifies the code by
removing cumulative_offset_for_frame_with_id() methods and storing
only own_offset values in ScrollStateSnapshot.
With this change we save a copy of of scroll state at the time of
recording a display list, instead of actual ScrollState pointer that
could be modifed by the main thread while display list is beings
rasterized on the rendering thread, which leads to a frame painted with
inconsistent scroll state.
Fixes https://github.com/LadybirdBrowser/ladybird/issues/4288
