cua-driver/Windows: Tauri AX foreground Enter misses the fixture keyboard input

[f-trycua]

Problem

On Windows Server 2022 build 20348 in an interactive Cua Cloud Fleet desktop, accessibility-targeted foreground Enter fails to reach the canonical Tauri fixture. The same pixel-targeted action passes. This reproduced in the complete Windows harness and a focused repeat at source SHA bb90abb62ed949451eb2a4aac11ee794677d0c96, Cua Driver source version 0.23.2.

Reproduction

1. Build that candidate and the repository desktop fixtures.
2. Run the canonical Windows harness: scripts\ci\windows\run-rust-e2e.ps1 -RequireGui.
3. Inspect windows-tauri-press-key-ax-foreground: obtain a fresh accessibility token for the fixture's keyboard input and invoke foreground press_key with Enter.
4. Verify the fixture event journal and a fresh snapshot. Expected: key_state=enter. Actual: key_state=none.
5. Repeat the four foreground Tauri keyboard cells with fresh fixture processes. AX Enter fails again; PX Enter and both AX/PX hotkeys pass.

The action returns isError:false, route global_input, transport windows_send_input, and effect unverifiable. The independent fixture oracle detects the missing delivery; the result does not claim a confirmed effect.

Evidence and scope

• Complete canonical run: 133 passing cells (107 delivered, 26 expected refusals), three failures, no skips. Installed-history validation passed.
• Focused repeat: eight passing cells and one failure across four Tauri keyboard cells and five desktop-scope cells. The remaining failure is AX Enter.
• The full run also failed Tauri AX foreground hotkey and WPF desktop foreground hotkey. Both passed the focused repeat, so those observations remain intermittent and are not established as the same root cause.
• Modern MCP direct/proxy desktop-action and resource probes passed independently. This issue blocks a green Windows desktop certification for feat(cua-driver): support modern stdio MCP and skills resources #3609; it does not establish that the MCP changes introduced the input defect.

Candidate investigation: Windows focus_cached_element_for_foreground / send_key_synthesized_after_focus, UIA child focus, and renderer delivery after scoped foreground activation. Root cause is unresolved. Related foreground contract: #2206; prior implementation: #3068.

Acceptance evidence

• Make the canonical Tauri AX foreground Enter cell deliver reliably, with the fixture-owned journal and fresh state as the oracle.
• Preserve PX Enter, AX/PX hotkey, and desktop-scope behavior; investigate the two intermittent hotkey failures.
• Verify focused fresh-process repeats and the affected canonical Windows harness at the final candidate SHA. Keep effect reporting truthful when delivery cannot be verified.