Huh, Gecko clobbers the border-radius on the scrollbar side when showing scrollbars (and gets it right in RTL). Weird.
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GIF
Pen: codepen.io/smfr/pen/rPwwrx

Overflow scrollers and border-radius

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codepen.io
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Replying to @davidbaron @smfr
And it looks like I implemented that in bugzil.la/459144 and the relevant spec prose is in drafts.csswg.org/css-backgro… ("If the curve interferes with UI elements such as scrollbars, ...")
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I'd note that back then, substantially more scrollbars had buttons at the ends than do today... and you really don't want to clip the buttons.