[bug] WebView2 deadlock on ARM64 (and potential reentrancy issue on all architectures)

[npiesco]

Describe the bug

Windows ARM64 (tested on my Snapdragon X Elite / aarch64-pc-windows-msvc), creating second WebView2 controller from main STA thread deadlocks inside MsgWaitForMultipleObjectsEx. Completion callback for CreateCoreWebView2Controlleris never delivered because nestedGetMessage/PeekMessageloop inwebview2_com::wait_with_pump` prevents COM from re-entering apartment.

This is root cause behind issue #583 (deadlock creating new window from IPC handler). #583 was worked around by creating windows outside handlers, underlying problem remains: mpsc::channel + wait_with_pump pattern uses nested Win32 message pump that does not allow COM STA re-entrancy.

Root Cause

wait_with_pump calls MsgWaitForMultipleObjectsEx + PeekMessage/DispatchMessage in a loop. This is a Win32-level message pump, not COM-level one. COM's STA requires that apartment's message filter processes incoming COM calls raw GetMessage loop does not do this. On x86/x64, this happens to work most of the time because WebView2's internal RPC dispatch piggybacks on window messages. On ARM64, timing/threading model exposes bug deterministically.

Fix

Replace mpsc::channel + wait_with_pump with CoWaitForMultipleHandles using COWAIT_DISPATCH_CALLS | COWAIT_DISPATCH_WINDOW_MESSAGES. This is COM-sanctioned mechanism for yielding an STA thread:

• Dispatches COM calls (re-entrancy) required for WebView2 async completion callbacks
• Dispatches window messages required for event loop to stay responsive
• Works on all architectures

Consistent with Microsoft's WebView2 threading model documentation on reentrancy.

Affected call sites

1. create_environment() CreateCoreWebView2EnvironmentWithOptions
2. create_controller() CreateCoreWebView2Controller / CreateCoreWebView2ControllerWithOptions
3. cookies_inner() CookieManager().GetCookies

Minimal Reproduction

https://github.com/npiesco/wry-arm64-deadlock

• main branch: deadlocks on ARM64 (uses upstream wry)
• fix/vendored-wry-tauri branch: works (uses patched wry fork)

Expected behavior

WebView2 environment and controller creation should complete without deadlocking on all architectures.

Platform and Versions

• OS: Windows 11 ARM64 (Snapdragon X Elite)
• Rustc: 1.87.0 (aarch64-pc-windows-msvc)
• wry: 0.54.1

Related

• Deadlock creating a new window on the IPC handler on Windows #583 Deadlock creating a new window on the IPC handler on Windows
• https://learn.microsoft.com/en-us/microsoft-edge/webview2/concepts/threading-model#reentrancy