1228879 - spend lots of time in nsRegion::Or called from ContainerState::FinishPaintedLayerData, with lots of polygons in a leaflet map

Status: NEW

(Core :: Web Painting, defect)

Description

[David Baron :dbaron: (⌚️UTC-4, no longer working on Mozilla)]

I was profiling the leaflet-osm-conversion branch of https://github.com/dbaron/timezone-map to figure out why it was slow in Gecko.  One thing I noticed is that we're spending significant amounts of time (>10%) in nsRegion::Or called from ContainerState::FinishPaintedLayerData.

This appears to be to union into containingPaintedLayerData->mDispatchToContentHitRegion and containingPaintedLayerData->mMaybeHitRegion, although it might also be including the calls through nsLayoutUtils::TransformToAncestorAndCombineRegions.

Comment 1

[Timothy Nikkel (:tnikkel)]

We should maybe be simplifying these regions, but it's not as straightforward as the other places that we simplify regions.

We don't want to expand mHitRegion (otherwise the compositor will falsely report a hit) so maybe would could SimplifyInward the hit region, and put area that SimplifyInward discards into the maybe hit region or dispatch to content region.

The maybe hit region and dispatch to content region could be simplified outward, but we are trading off having to ask the main thread about taps for a simpler region here.

Comment 2

[Timothy Nikkel (:tnikkel)]

ContainerState::FinishPaintedLayerData and related code no longer exists. We should compare/profile this with current code and see if there is any issue.