+++ This bug was initially created as a clone of Bug #1403483 +++ See screenshot (attachment 8912611 [details]), the first tab separator should have the same height as the other separators. We forgot to do that in bug 1390025. +++ End bug description copied from bug 1403483 +++ I'm filing this bug as a clone so we can quickly land an independent patch for 57, since the patch for 58 will be based on top of the patch from bug 1391539 which is not being uplifted.

Comment on attachment 8919416 [details] Bug 1408953 - Set border at the start of the first tab instead of at the end of the pre-tabs whitespace. https://reviewboard.mozilla.org/r/190238/#review195594 There are some issues with this patch: - If the first tab is selected, it should still have a separator in the tabs-border color on its left hand side (easiest to spot in light theme). - (More importantly) when dragging a tab to the beginning of the tabstrip, it resizes slightly. I'm not sure how to solve this given your current approach and that is making me worried that there might be other subtle bugs beneath the surface. Sorry, contrary to what I said on IRC before, I'm not sure there's a way we can uplift this while removing the border from the placeholder and putting it on the tab.

I've uploaded a new patch that addresses the issues you brought up, but you're probably right about not being able to uplift this. I was tempted to WONTFIX this bug right now, but I decided to wait for you to take a quick look at the new patch and agree for sure that we can't uplift. :)

Comment on attachment 8919416 [details] Bug 1408953 - Set border at the start of the first tab instead of at the end of the pre-tabs whitespace. https://reviewboard.mozilla.org/r/190238/#review196294 While the patch in general seems fine to me, I don't think it's worth the risk of uplifting this. That, in turn, means we can spend more time looking for a better solution (maybe involving permanently showing both pseudo-elements). So I'd vote for WONTFIXing this bug. I hope that's ok for you.