The tab strip used flex: 1, which stretched it to fill the popup width.
scrollWidth then equaled clientWidth, so each ResizeObserver pass set the
popup wider, which stretched the strip wider, and so on until the screen
clamp. Switching to flex: 0 1 auto + min-width: 0 lets scrollWidth report
intrinsic content width while still allowing the strip to shrink and
scroll horizontally if the popup is forced narrower than its tabs.
David