Diagnostic page
Position modes
The coordinateSpace option controls how the popup is
positioned. Viewport (default): viewport coordinates +
fixed, so the picker stays attached while the browser
scrolls. Document (opt-in): document coordinates +
absolute, an explicit optimization for references known to
move with root-page scrolling — a stuck sticky anchor does not, so the
picker visibly trails until JavaScript repositions it. No layout
inference is performed. The value is read once per opening; a change
while open applies to the next opening.
Viewport: not measured yet.
Document: not measured yet.
Only one picker is ever open at a time: the comparison runs sequentially (open → scroll → measure → settle → close, then the other widget at the exact same scroll position), respecting the single-open-picker rule. The immediate reading happens deliberately before the next animation frame to expose the coordinate mismatch; both modes converge afterwards. Mouse-wheel scrolling may make the trailing barely noticeable; asynchronous touch scrolling is where the problem becomes visible.
Fallback: positioning is enhanced-mode only. Without enhancement both widgets degrade to plain native selects.