Implement real popup selection for collapsed HTML select controls #44

Current implementation checkpoint — 20 September 2026

The WebScene-rendered collapsed-select popup architecture and core implementation are merged through PR #66. Current WebScene main is 947e9b7726674d4e15b119a184fbc85a0c575457. No #44 implementation branch is active.

Remaining work is broader acceptance and follow-up: browser-sensitive focus/keyboard details, long-list/RTL/transform/viewport mutation cases, unchanged Kestrel behavior, composition above GPU/video, native accessibility exposure, and the optional AppScene popup-window host when out-of-surface placement is required. The same WebScene-rendered popup must remain authoritative.

Problem

Collapsed HTML <select> controls do not open a dropdown in WebScene. This is visible in the unchanged Kestrel CAD sample. Clicking currently cycles immediately to the next option, preventing users from inspecting and choosing from a list.

This is a general form-control compatibility gap, separate from WebGPU implementation in epic #22. Track it separately so it does not interrupt GPU qualification.

Accepted architecture

PR #66 records and implements the agreed direction:

• standard <select>/<option>/<optgroup> remain the application API;
• WebScene owns select behavior and dropdown rendering;
• dropdown presentation is WebScene-rendered HTML/CSS with native C++ behavior;
• popup hosting is separate from popup rendering;
• the default host is an in-surface WebScene overlay;
• a future AppScene/platform popup host may place the same WebScene-rendered popup into a real OS popup window, but that window is only a container for a WebScene rendering surface, not an OS-native dropdown widget;
• native control presenters remain a separate option, especially for menus/context menus and other controls where platform-native integration is itself desirable.

Architectural rule: hosting may be native; dropdown rendering remains WebScene-rendered.

Current implementation

The implementation in PR #66 removes the click-to-cycle fallback for collapsed single selects and adds an engine-owned in-surface popup. The popup uses a closed private shadow tree, mirrors authoritative option state without replacing the authored options, and renders through the normal WebScene scene/layout pipeline.

The implementation includes pointer selection, pending keyboard highlight with explicit commit, Escape/outside cancellation, disabled options/optgroups, scrolling, basic type-ahead, viewport placement, option-identity commits, and stale option-state detection. Input/change are emitted only when committed option identity actually changes.

The existing Web Platform contract and native runtime regression are changed from click-to-cycle to open/commit/cancel behavior. The native regression also checks disabled-optgroup keyboard navigation and that outside-click dismissal does not leak the pointer gesture into content underneath.

Remaining acceptance / follow-up

Keep this issue open until broader qualification is complete:

1. Qualify browser-sensitive keyboard/event details and focus behavior across supported desktop platforms.
2. Cover long-list scrolling, RTL, transforms, viewport edges, resize/display-scale changes and mutation/reentrancy cases comprehensively.
3. Verify unchanged Kestrel dropdowns can open and choose non-adjacent values and drive expected CAD behavior.
4. Verify composition above WebGPU/video does not disturb image retirement/presentation.
5. Implement and verify the native accessibility bridge for the owning select/listbox semantics.
6. Add the optional AppScene/platform popup-window host when out-of-surface presentation is required; it must host the same WebScene-rendered popup rather than replace it with an OS-native dropdown.
7. Keep multiple/size > 1 listbox semantics distinct.

Do not close based solely on a static popup screenshot, request parser, or programmatic value assignment.