Motivation: some horizontally scrollable slider by flex in a grid cell. AR: The resized <img> allocates the original size of the image. ER: Like this fiddle: https://jsfiddle.net/9eb69gye/16/ but images might be in a wrapper. Although the image resized to the grid-cell's height, still allocates the original image width in the cell. Crossbrowser compat: chromium looks ok by decreasing the width, although has other bug, scrollbar overlaps the content in Win10. Testcase attached. in the testcase the grid cell is sized to 100px, but should be work with 1fr value as well.

I'm betting that something like this is what's going on here: (a) the flexbox (#slides) isn't considering its height to be "definite", so it doesn't proactively stretch (shrink) its items (the images) in the cross axis. (b) ...so we resolve the items' main size to be the image's intrinsic size (kinda under the assumption that the image will actually get its inrinsic height) (c) ...but then in the final layout, it turns out the flex container *does* have a constrained height after all, so the images do as well, so their width is constrained. But we've already determined their x-positions without that knowledge. This is likely related to the spec change (or at least the spec text) in bug 1055354. Probably best to fix this as part of sorting that issue out. Marking as dependency.