Radix Dialog's modal DismissableLayer calls preventDefault() on pointerdown
events originating outside the Dialog DOM tree. Popover portals render at the
body level (outside the Dialog), so touch events on popover content were
treated as 'outside' — killing scroll gesture recognition on mobile.
Fix: add onPointerDownOutside to NewIssueDialog's DialogContent that detects
events from Radix popper wrappers and calls event.preventDefault() on the
Radix event (not the native event), which skips the Dialog's native
preventDefault and restores touch scrolling.
Also cleans up previous CSS-only workarounds (-webkit-overflow-scrolling,
touch-pan-y on individual buttons) that couldn't override JS preventDefault.
c486bad2dd