fix(cua-driver-rs)(get_window_state): 30s timeout + 2000-node cap for heavy webview apps

[f-trycua]

Summary

Closes #1537. Cherry-picked from #1609 (@hippoley) — same fix concept, ported to the post-#1674 path layout (libs/cua-driver/rust/crates/).

When get_window_state walks the AX tree of a pathologically large app (Arc, Safari with many tabs, Electron), AXUIElementCopyAttributeValue can block indefinitely via XPC. There's no built-in deadline, so the MCP server just hangs forever — reported by @obaid on macOS 26.

Two changes, both Rust-only:

• libs/cua-driver/rust/crates/platform-macos/src/ax/tree.rs — MAX_ELEMENTS = 2000 cap on total nodes visited in a single walk. walk_element now threads a shared visited_count + truncated flag; when the cap is hit the walk stops early and TreeWalkResult.truncated = true. The renderer appends a warning line to tree_markdown so the agent knows the partial tree is intentional and existing element indices remain valid.
• libs/cua-driver/rust/crates/platform-macos/src/tools/get_window_state.rs — wraps the spawn_blocking AX walk in tokio::time::timeout(Duration::from_secs(30)). On expiry returns a descriptive ToolResult::error suggesting capture_mode=vision or a query filter rather than hanging.

Notes

• Swift backend changes from fix(get_window_state): 30s timeout + 2000-node cap for heavy webview apps #1609 (libs/cua-driver/Sources/CuaDriverCore/AppState/AppState.swift) deliberately dropped — that path is being deprecated per [codex] docs(cua-driver): pivot to rust-first driver #1738.
• TreeWalkResult gains a new public field (truncated: bool); additive, doesn't break existing callers.
• Credit to @hippoley — the 30s timeout + 2000-node cap shape is verbatim from their PR. Co-authored.

Test plan

• cargo build --release -p cua-driver on macOS — green
• cargo check -p platform-windows --target=x86_64-pc-windows-msvc — green (additive struct field doesn't touch Windows code)
• Live repro on Arc / Safari w/ many tabs (the issue get_window_state hangs forever on heavy webview apps (Arc, Safari w/ many tabs) on macOS 26 #1537 repro): verify get_window_state returns a truncation warning rather than hanging
• Verify the 30s timeout fires with a clear error message on a known-bad target

cc @obaid for the original repro, @hippoley for the original fix.

Summary by CodeRabbit

• Bug Fixes
  • Added performance safeguards to prevent hangs when examining complex accessibility interfaces.
  • Implemented timeout protection (30 seconds) for accessibility tree operations with improved error guidance.
  • Enhanced reliability of accessibility tree traversal operations.

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Project	Deployment	Actions	Updated (UTC)
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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 048976a4-3966-4707-8d74-03b79943bca7

📥 Commits

Reviewing files that changed from the base of the PR and between 8a77923 and f1390ee.

📒 Files selected for processing (2)

• libs/cua-driver/rust/crates/platform-macos/src/ax/tree.rs
• libs/cua-driver/rust/crates/platform-macos/src/tools/get_window_state.rs

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📝 Walkthrough

Walkthrough

The PR hardens AX tree traversal against indefinite hangs on heavy webview apps by capping the total node visits at 2,000 and wrapping the walk in a 30-second timeout. The walker now tracks traversal progress and stops early with a truncation flag, while callers receive explicit timeout guidance instead of hanging forever.

Changes

AX Tree Traversal Protection

Layer / File(s)	Summary
AX tree node visit cap and truncation tracking libs/cua-driver/rust/crates/platform-macos/src/ax/tree.rs	MAX_ELEMENTS constant limits total visited nodes; TreeWalkResult gains a truncated: bool field; walk_tree initializes shared visited_count and truncated state passed to walk_element; walk_element enforces early-exit when cap is reached and sets the truncation flag; all recursive calls to walk_element propagate the cap state to halt traversal across the entire depth-first walk; truncation warning is appended to output when limit is hit.
30-second timeout on AX tree walk libs/cua-driver/rust/crates/platform-macos/src/tools/get_window_state.rs	tokio::time::timeout wrapper replaces unconditional await on walk_tree blocking task; timeout elapsed returns error with guidance to switch capture_mode; walk failure returns underlying error message.

🎯 2 (Simple) | ⏱️ ~12 minutes

🐰 A rabbit bounces past the AX tree so tall,
Says "Let's cap this walk at two-thousand nodes, that's all!"
With a timeout of thirty, no more endless wait,
The apps won't hang heavy—we're fixing their fate! 🌳⏱️

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/get_window_state-timeout-element-cap

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