Fix fullscreen focus with native file dialogs

[volksec]

Summary

Prevent the Windows viewer from minimizing in exclusive fullscreen when focus moves to a native file dialog owned by the same process.

Fixes #4569.

Root cause

Native file dialogs can run on a worker thread. When focus moves between the viewer window and that dialog, WM_ACTIVATEAPP reports deactivation and the fullscreen handling minimizes the viewer, even though the newly active thread belongs to the viewer process.

Changes

• Determine whether the thread receiving activation belongs to the current process.
• Keep the application active for same-process focus transitions.
• Preserve the existing minimize behavior when focus moves to another application.

Validation

• git diff --check
• cmake --build build-vc170-64 --config Release --target llwindow -- /m
• cmake --build build-vc170-64 --config Release --target secondlife-bin -- /m

Both llwindow.lib and secondlife-bin.exe were built successfully with Visual Studio 2022. Manual reproduction in exclusive fullscreen is still pending.

[Copilot]

Pull request overview

This PR adjusts Windows fullscreen focus handling to prevent the viewer from minimizing when focus moves between the main window and a native file dialog owned by the same process (e.g., dialogs running on worker threads), addressing issue #4569.

Changes:

• Add a helper to determine whether a thread ID belongs to the current process.
• Treat WM_ACTIVATEAPP “deactivation” as still-active when activation moves to a same-process thread, avoiding fullscreen minimize/hide-dialog behavior.

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