[Peek] Stop fail-fast in AppWindow.Closing path; reset cached preview-handler factories on release

[crutkas]

Summary

Harden Peek's AppWindow.Closing path so a stale cached preview-handler factory can't fail-fast the Peek process. Also clean up the matching path in RegistryPreview.

Background

Spotted while reading through Peek's MainWindow teardown sequence and the ShellPreviewHandlerPreviewer cache for an unrelated review of how Peek manages out-of-process preview-handler lifetimes.

The Peek MainWindow subscribes to AppWindow.Closing. The handler doesn't actually close the window — it sets args.Cancel = true and calls Uninitialize(), which in turn calls ShellPreviewHandlerPreviewer.ReleaseHandlerFactories().

ReleaseHandlerFactories() looked like this:

public static void ReleaseHandlerFactories()
{
    foreach (var factory in HandlerFactories.Values)
    {
        try { Marshal.FinalReleaseComObject(factory); } catch { }
    }
}

Two problems:

1. The static HandlerFactories dictionary is never cleared. After FinalReleaseComObject, the entries still point at separated RCWs. A subsequent activation that races with this cleanup (or a second close in the same process) can pick up the dead RCW from the cache.
2. The cached factory had LockServer(true) called on it when it was first cached, but the matching LockServer(false) was never paired.

Any managed exception that escapes a WinRT event callback is projected back to CFlat as a failed HRESULT and the CsWinRT dispatcher fail-fasts the process. So a single InvalidComObjectException (HRESULT 0x80131527) thrown out of Uninitialize() is enough to terminate Peek.

Changes

• ShellPreviewHandlerPreviewer.ReleaseHandlerFactories — snapshot then clear the dictionary up front so that a subsequent call (or a concurrent LoadPreviewAsync) can't pick up a stale RCW. Call LockServer(false) before FinalReleaseComObject to mirror the cache-time LockServer(true). Both COM calls remain individually wrapped because the RCW may already be unreachable during process teardown.
• Peek.UI/MainWindow.xaml.cs — AppWindow_Closing — wrap the body in try/catch + Logger.LogError. Any future exception in Uninitialize() (or its callees) will now log instead of fail-fasting the process.
• RegistryPreview/MainWindow.Events.cs — AppWindow_Closing — same defensive try/catch, plus null-guard jsonWindowPlacement before SetNamedValue. The placement dictionary can legitimately be null on first run or after a corrupt placement file; previously that would NRE → fail-fast.

Risk

Low. The ReleaseHandlerFactories change matches the documented LockServer/FinalReleaseComObject pairing and only widens the lifetime window of the cache by Clear()-ing earlier; nothing in Peek calls this method outside of teardown. The two try/catch wrappers strictly add defense — the success path is unchanged.

Validation

Spot-built locally; this repo's dotnet restore runtime-pack issue (unrelated to this PR — same NU1102 pattern that's affecting other open PRs) prevents a full Build.cmd here. The C++ side of Peek is untouched.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

---

ADO: https://microsoft.visualstudio.com/DefaultCollection/OS/_workitems/edit/58765809/

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