There is an issue with RTL scrollbar rendering which may have been introduced in part by bug 1423013. `UpdateMinimumScaleSize()` is [sizing the layout viewport based on the `XMost()` and `YMost()` of the scrollable overflow rect](https://searchfox.org/mozilla-central/rev/00c0d068ece99717bea7475f7dc07e61f7f35984/layout/generic/nsGfxScrollFrame.cpp#5576), rather than its width and height. On RTL scrollframes, that can result in a layout viewport that's too small. With containerless scrolling, this then breaks scrollbar rendering because we apply a clip sized to the layout viewport to the scrollframe contents, and this can clip away the scrollbars. The too-small layout viewport is likely to cause other problems with container scrolling too. This causes the following tests to fail with containerless scrolling enabled: layout/reftests/bugs/1133905-4-vh-rtl.html layout/reftests/bugs/1133905-5-vh-rtl.html layout/reftests/bugs/1133905-6-vh-rtl.html In bug 1158392 I am going to temporarily disable these tests, so that we can enable containerless scrolling in nightly, but I plan to re-enable in this bug before letting containerless scrolling ride the trains.
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There is an issue with RTL scrollbar rendering which may have been introduced in part by bug 1423013. `UpdateMinimumScaleSize()` is [sizing the layout viewport based on the `XMost()` and `YMost()` of the scrollable overflow rect](https://searchfox.org/mozilla-central/rev/00c0d068ece99717bea7475f7dc07e61f7f35984/layout/generic/nsGfxScrollFrame.cpp#5576), rather than its width and height. On RTL scrollframes, that can result in a layout viewport that's too small. With containerless scrolling, this then breaks scrollbar rendering because we apply a clip sized to the layout viewport to the scrollframe contents, and this can clip away the scrollbars. The too-small layout viewport is likely to cause other problems with container scrolling too. This causes the following tests to fail with containerless scrolling enabled: layout/reftests/bugs/1133905-4-vh-rtl.html layout/reftests/bugs/1133905-5-vh-rtl.html layout/reftests/bugs/1133905-6-vh-rtl.html (These are the test pages that load at zoom levels greater than 1.) In bug 1158392 I am going to temporarily disable these tests, so that we can enable containerless scrolling in nightly, but I plan to re-enable in this bug before letting containerless scrolling ride the trains.