It looks to me like the page is doing something (replacing `document.documentElement` and such) that is causing `mIsRootContent` to change from `true` to `false`. I think this is a case that APZ does not expect outside of the [first-paint case](https://searchfox.org/mozilla-central/rev/0bcf81557b89e7757c44e25bb4bc7f4cb8619dc9/gfx/layers/apz/src/AsyncPanZoomController.cpp#4717). One option here would be to take the first-paint branch if `mIsRootContent` has changed. Another option would be to just update `mIsRootContent` in the other branch (where we [update the other fields](https://searchfox.org/mozilla-central/rev/0bcf81557b89e7757c44e25bb4bc7f4cb8619dc9/gfx/layers/apz/src/AsyncPanZoomController.cpp#4820)), though I wonder if there may be other state tied to `mIsRootContent` that APZ does not expect to change.
Bug 1681955 Comment 2 Edit History
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It looks to me like the page is doing something (replacing `document.documentElement` and such) that is causing `mIsRootContent` to change from `true` to `false`. I think this is a case that APZ does not expect outside of the [first-paint branch](https://searchfox.org/mozilla-central/rev/0bcf81557b89e7757c44e25bb4bc7f4cb8619dc9/gfx/layers/apz/src/AsyncPanZoomController.cpp#4717). One option here would be to take the first-paint branch if `mIsRootContent` has changed. Another option would be to just update `mIsRootContent` in the other branch (where we [update the other fields](https://searchfox.org/mozilla-central/rev/0bcf81557b89e7757c44e25bb4bc7f4cb8619dc9/gfx/layers/apz/src/AsyncPanZoomController.cpp#4820)), though I wonder if there may be other state tied to `mIsRootContent` that APZ does not expect to change.