ArrowDown on a picker that has never been opened pins the deferred path where
_tree is still null when the keydown is handled: the panel opens in that render
pass and the roving tab stop lands on the first row in the next one. Escape on a
closed picker is pinned as a no-op.
Two notes, no behaviour change: FirstVisibleInstance records WHY it may trust
TreeView.IsExpanded (Blazor Server's single renderer sync context runs the render
plus the pending ExpandAll/CollapseAll before the next keydown is dispatched — an
await that genuinely yields inside OnAfterRenderAsync would break that), and the
stylesheet warns integrators that the absolutely-positioned panel is clipped by
any ancestor with overflow: hidden/auto.
The ArrowDown focus interop assertion is kept — it was stable across repeated
runs — with a comment marking the tabindex as the behavioural check.
Claude-Session: https://claude.ai/code/session_01NcMG6xXdsJbmD2wYAgkK29
Joseph Doherty
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