Describe the bug:

assertion `left == right` failed
  left: SpatialId(2, PipelineId(1, 1))
 right: SpatialId(1, PipelineId(1, 1)) (thread Script(1,1), at components\layout\display_list\mod.rs:565)

To Reproduce:

1. Save example in Unspecified overflow hides item from hit-test #40663 (comment) as hit_test_z_index.html
2. ./mach run -r <path to above file> --devtools=6080
3. open about:debugging in Firefox, connect and inspect.
4. Hover over <div> like below.

5. Bang.