The dropdown was clipped by parent containers using overflow, transform,
or stacking context via shadow-sm + z-index combinations. Adopts the same
fixed-position strategy as PersonMultiSelect: binds to the input element,
computes position via getBoundingClientRect(), and registers svelte:window
scroll/resize listeners to keep it current.
Also adds full ARIA combobox pattern (role=combobox, aria-expanded,
aria-haspopup, aria-controls, aria-activedescendant) and keyboard
navigation (ArrowDown/Up, Enter, Escape) matching TagInput's reference
implementation.
Removes the now-dead z-30/z-10 z-index workarounds from ConversationFilterBar.
Closes#343
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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