RenderLayer::invalidateEventRegion() and RenderLayerBacking::updateEventRegion() only generate an event region for non-root layers so the UI process has more info for hit testing.
This bug came up while considering alt patch attachment #395855 [details] (bug 210041). It removed an iOS compile-time guard from RenderLayer::invalidateEventRegion(), which made causes this progression: [[ --- /Volumes/Data/worker/macOS-Mojave-Release-WK2-Tests-EWS/build/layout-test-results/fast/scrolling/mac/event-region-subframe-expected.txt +++ /Volumes/Data/worker/macOS-Mojave-Release-WK2-Tests-EWS/build/layout-test-results/fast/scrolling/mac/event-region-subframe-actual.txt @@ -6,6 +6,9 @@ (GraphicsLayer (bounds 800.00 600.00) (contentsOpaque 1) + (event region + (rect (0,0) width=800 height=600) + ) (children 1 (GraphicsLayer (position 8.00 8.00) ]]