Bugzilla

So here are my thoughts on the the changes in test behaviour:

* The tests concern scrollbar rendering behaviour.
* The changes in behaviour are caused by the first patch, likely due to taking [different codepaths in `ScrollFrameHelper::LayoutScrollbars()`](https://searchfox.org/mozilla-central/rev/7d7aca6a935f0725f35050fdaa3dd7a1b05d8d38/layout/generic/nsGfxScrollFrame.cpp#5710).
* The patch in question only affects the configuration where we use `MobileViewportManager` but `apz.allow_zooming=false`. This is the case for Android reftests as mentioned above, but not actual production behaviour on Android, where we run with `apz.allow_zooming=true`. Therefore, this patch should not affect production behaviour on Android.
* This configuration is, however, used in RDM (after all, that's the point of the patch), so it may alter scrollbar rendering behaviour in RDM.
* The changes to the test expectations suggest that the alterations to scrollbar rendering behaviour may include both regressions (as some tests start to fail) and improvements (as some tests start to pass).
* As this patch fixes a fairly significant regression in RDM behaviour, I think it's reasonable to take it even though it may cause some regressions to scrollbar rendering behaviour, which seems less severe.

I do think it would be a good idea to investigate and fix the test cases that are still failing, as a follow-up. We already have bug 1308702 on file for the bug 1133905 test failures on Android, so we can use that to track.

So here are my thoughts on the the changes in test behaviour:

* The tests concern scrollbar rendering behaviour.
* The changes in behaviour are caused by the first patch, likely due to taking [different codepaths in `ScrollFrameHelper::LayoutScrollbars()`](https://searchfox.org/mozilla-central/rev/7d7aca6a935f0725f35050fdaa3dd7a1b05d8d38/layout/generic/nsGfxScrollFrame.cpp#5710).
* The patch in question only affects the configuration where we use `MobileViewportManager` but `apz.allow_zooming=false`. This is the case for Android reftests as mentioned above, but not actual production behaviour on Android, where we run with `apz.allow_zooming=true`. Therefore, this patch should not affect production behaviour on Android.
* This configuration is, however, used in RDM (after all, that's the point of the patch), so it may alter scrollbar rendering behaviour in RDM.
* The changes to the test expectations suggest that the alterations to scrollbar rendering behaviour may include both regressions (as some tests start to fail) and improvements (as some tests start to pass).
* As this patch fixes a fairly significant regression in RDM behaviour, I think it's reasonable to take it even though it may cause some regressions to scrollbar rendering behaviour in RDM, which seems less severe.

I do think it would be a good idea to investigate and fix the test cases that are still failing, as a follow-up. We already have bug 1308702 on file for the bug 1133905 test failures on Android, so we can use that to track.