I discovered in bug 1656689, comment 16 that the iframe that the mochitest harness uses to run tests in has scrolling="no" set on it, here https://searchfox.org/mozilla-central/rev/7ef5cefd0468b8f509efe38e0212de2398f4c8b3/testing/mochitest/server.js#824 This is different from almost every other document that we ever see, so we shouldn't be doing this for tests. One place this effects is ScrollFrameHelper::GetScrollStylesFromFrame https://searchfox.org/mozilla-central/rev/7ef5cefd0468b8f509efe38e0212de2398f4c8b3/layout/generic/nsGfxScrollFrame.cpp#4429 I noticed the same thing about the mochitest-chrome harness in bug 1199023, but this uses an unusually way to add scrolling=no that would be hard to search for. I tried to look up the history for this, it seems to be back to 2007 when the mochitest harness was added. We fail a handful of tests when this is removed that will have to be fixed in order to fix this https://treeherder.mozilla.org/#/jobs?repo=try&revision=a50f3facfb0e1236a92c295dc9ef41d584229d6b
Bug 1668969 Comment 0 Edit History
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I discovered in bug 1656689, comment 16 that the iframe that the mochitest harness uses to run tests in has scrolling="no" set on it, here https://searchfox.org/mozilla-central/rev/7ef5cefd0468b8f509efe38e0212de2398f4c8b3/testing/mochitest/server.js#824 This is different from almost every other document that we ever see, so we shouldn't be doing this for tests. One place this effects is ScrollFrameHelper::GetScrollStylesFromFrame https://searchfox.org/mozilla-central/rev/7ef5cefd0468b8f509efe38e0212de2398f4c8b3/layout/generic/nsGfxScrollFrame.cpp#4429 I noticed the same thing about the mochitest-chrome harness in bug 1199023, but this uses an unusual way to add scrolling=no that would be hard to search for. I tried to look up the history for this, it seems to be back to 2007 when the mochitest harness was added. We fail a handful of tests when this is removed that will have to be fixed in order to fix this https://treeherder.mozilla.org/#/jobs?repo=try&revision=a50f3facfb0e1236a92c295dc9ef41d584229d6b