Created attachment 376451 [details] Repro case for bug We've seen a regression in a recent Safari release (not sure when it occurred) that prevents a web component of ours from rendering properly in response to user interaction. We've isolated a minimum repro case at https://jsbin.com/xariwib/edit?html,output (or see attachment). Expect: Clicking the buttons in the demo toggles their "selected" class, which ends up applying :host styling to show their selection state. This works as expected in Chrome and Firefox. Actual: in Safari 12+, the buttons begin to move or disappear.
<rdar://problem/54420420>
I can still reproduce this with trunk WebKit.
This appears to be fixed now (as of Safari 13.1)