STR: 0. Flip layout.css.contain.enabled = true 1. Load attached testcase ACTUAL RESULTS: The black border stretches to wrap the text in the 2nd, 3rd, and 4th boxes. EXPECTED RESULTS: Black border should *not* stretch to wrap the text. It should just collapse to a single point (perhaps with visible-but-useless scrollbars in the 3rd box -- the overflow:scroll one), as if there were no contents. Chrome gives "expected results" (without any visible scrollbars on the 3rd box, FWIW - not sure if that's a chrome bug or not). (Thanks to Gerard for bringing this issue up in bug 1478550 comment 17.)

I suspect the cleanest way to fix this would be to adjust nsHTMLScrollFrame::ReflowContents [1] or nsHTMLScrollFrame::ReflowScrolledFrame [2] or some code around there to test whether we have contain:size, and to be sure to ignore our scrolled frame's size when determining our own "outer" scroll-frame size. [1] https://dxr.mozilla.org/mozilla-central/rev/4e56a2f51ad739ca52046723448f3129a58f1666/layout/generic/nsGfxScrollFrame.cpp#698-701 [2] https://dxr.mozilla.org/mozilla-central/rev/4e56a2f51ad739ca52046723448f3129a58f1666/layout/generic/nsGfxScrollFrame.cpp#482-487

(In reply to Daniel Holbert [:dholbert] from comment #0) > Chrome gives "expected results" (without any visible scrollbars on the 3rd > box, FWIW - not sure if that's a chrome bug or not). (I filed https://bugs.chromium.org/p/chromium/issues/detail?id=872794 on this Chrome side-issue, BTW.)

This is a partial patch I've written with dholbert's test included. The patch reduces the BSize of overflow:[hidden | auto] elements to zero. It doesn't address overflow:scroll elements. Tests to add might include: * elements with contain:size, an overflow property, and fixed height * elements with contain:size, an overflow property, and fixed width * elements with contain:size, an overflow property, and fixed height and width for all three overflow variants. It's probably easiest to grab one of the contain-size-block* or contain-size-fieldset* reftests as a boilerplate, since may of those test cases can have overflow:[hidden | auto | scroll] added.

2 tests with 'contain: size; overflow: hidden': http://test.csswg.org/harness/test/css-contain-1_dev/single/contain-size-011/format/html5/ http://test.csswg.org/harness/test/css-contain-1_dev/single/contain-size-061/format/html5/