Fix #166: stop focus restore from fighting native desktop switching

[koppor]

Fixes the focus "fighting" from #166, specifically the mechanism identified in this comment: switching desktops natively snaps back to the previous desktop and refocuses the wrong window.

Root cause

With feature.restorePreviousWindowFocus enabled, every detected switch runs App._restorePrevWinFocus(), which SetForegroundWindow()s the window recorded for the desktop just landed on. That record comes from _storeLastWinFocused(), polled every 200 ms:

IntPtr hWnd = GetForegroundWindow();           // (A) reads window
...
var displayNumber = GetVDDisplayNumber(false); // (B) reads desktop number
VDDToLastFocusedWin[displayNumber] = hWnd;      // files A under B

(A) and (B) are read non-atomically, so during a native switch animation a window that lives on desktop 1 gets filed under desktop 2. Restoring it later force-focuses a window on another desktop, and Windows jumps to that desktop to show it — the snap-back. Rapid switches poison the map heavily, producing the erratic multi-hop behavior also reported.

Fix

Guard both sides with the public IVirtualDesktopManager::IsWindowOnCurrentVirtualDesktop:

• Store: only record a window under the desktop it actually lives on (no more poisoning).
• Restore: only SetForegroundWindow if the recorded window is still on the desktop just landed on (no snap-back even if a stale record slips through).

The public IVirtualDesktopManager (CLSID AA509086-…, IID A5CD92FF-…) is stable across all Win10 1607+/Win11 builds — important because affected users fall back to VirtualDesktopWin11_23H2_2921, which has no per-version manager. It's added once in Util.OS (fail-closed on any COM error), so the 9 version-specific implementations, the app's IVirtualDesktopManager interface, and the default settings are all untouched.

Scope

This addresses the focus-restoration fighting. The feature is off by default, so the separate "fighting on stock config" reports (feature disabled ⇒ no SetForegroundWindow on the switch path at all) are a different, Windows-level cause (wallpaper/accent-color repaint, stale post-sleep COM) and are out of scope here.

Testing

• Builds clean (VS 2026 MSBuild, .NET Framework 4.7.2).
• Reproduced the comment's scenario with the feature enabled (two desktops, two apps each, different focus per desktop): before → snaps back to the origin desktop; after → stays on the target desktop with its own last-focused window. Rapid switching no longer multi-hops.
• Ran ~1 hour of normal daily use with restorePreviousWindowFocus on — no snap-back observed.

No automated test: the path is COM- and live-desktop-bound and the repo has no test project.

🤖 Generated with Claude Code

[dankrusi]

hi @koppor , many thanks.

Am testing this now as well... Anyone on windows 10 tested this?

[koppor]

Am testing this now as well... Anyone on windows 10 tested this?

Yes. The PR was created on Windows 10 (although development took place on Windows 11).

[dankrusi]

@koppor What do you mean exactly the PR was created on Windows 10? Does that mean it was also thoroughly tested on Windows 10? I am not just talking about it building, but the actual app tested properly for any regressions (including cases like sleep/resume etc)

[koppor]

@koppor What do you mean exactly the PR was created on Windows 10? Does that mean it was also thoroughly tested on Windows 10? I am not just talking about it building, but the actual app tested properly for any regressions (including cases like sleep/resume etc)

My desktop machine is Windows 10, but has serious issues with suspend/resume, so I could not test there.

I see two options to move forward:

1. Build a test binary and ask other users to test / work with it and if there is no related issue then release in 1 or 2 months
2. Setup Sandbox Windows 10 VM and do a structured test with a checklist covering all the edge cases

[koppor]

Does that mean it was also thoroughly tested on Windows 10?

I am using it continuously on my Windows 10 desktop and Windows 11 laptop. Please consider it for the next release.