"Stalled Steam update half-deletes the s&box install"

The s&box editor silently fails to open projects — the launcher's "open project" button does nothing (no window, no error dialog). Headless dotnet build may also fail with CS0006 ("Metadata file could not be found") referencing engine DLLs. Everything worked yesterday.

If sbox-launcher.exe stays running across a Steam update (e.g. left open overnight), Steam downloads and stages the update but cannot commit it while files are in use. The install ends up in a half-applied state: key files are missing (sbox-dev.exe, random DLLs like Sandbox.CodeUpgrader.dll), but Steam considers the update complete.

How to diagnose

1. Check steamapps/appmanifest_590830.acf — look for StateFlags not equal to 4 (fully installed) while BytesStaged shows 100%.
2. Launcher errors appear only in sbox/logs/sbox-launcher.log, not on screen.
3. The dotnet build CS0006 errors point at engine DLLs in Base Library.csproj — the csprojs themselves are fine; the referenced DLLs are just gone from disk.

1. Kill the launcher — close sbox-launcher.exe completely.
2. Validate the install — run steam://validate/590830 (or right-click s&box in Steam → Properties → Installed Files → Verify integrity).
3. Wait for StateFlags to return to 4.
4. Relaunch normally.

Do not edit or "fix" csproj files — they were never broken. The missing DLLs are engine-shipped files that the stalled update removed but couldn't replace.

This presents as a mysterious build or editor failure with no obvious cause. The natural instinct is to debug csproj references or reinstall — but the root cause is a file-locking race between the launcher and Steam's update mechanism. Closing the launcher and validating restores every missing file.