Thanks for tackling this — the flashing console windows from git.exe grandchildren on the desktop build are a real annoyance (#2930), and the cross-platform guarding here is done right: hide_windows.go is behind //go:build windows with a !windows no-op stub, so Linux/macOS builds stay clean.

There's one blocker before I can merge, though. Adding DETACHED_PROCESS (0x00000008) alongside CREATE_NO_WINDOW regresses child-process stdout capture. The bash tool routes every command through setKillTree → proc.HideWindow, so the new flag lands on every command the agent runs, and the captured output comes back empty:

--- FAIL: TestBashPowerShellRunsNativeCommand
    bash_powershell_test.go:41: output = "", want it to contain reasonix-ok
--- FAIL: TestBashPowerShellSurfacesNonZeroExit
--- FAIL: TestBashPowerShellAllowsQuotedOperator
--- FAIL: TestBashPowerShellOutputIsUTF8
    bash_powershell_test.go:100: non-ASCII output mojibake — got "" (want it to contain 中文)

Root cause: DETACHED_PROCESS and CREATE_NO_WINDOW are conflicting console-disposition flags. DETACHED_PROCESS fully detaches the child from the console subsystem, which breaks the stdout/stderr pipe the tool relies on. CREATE_NO_WINDOW on its own already runs the child with no visible window — so dropping DETACHED_PROCESS and keeping just CREATE_NO_WINDOW (+ HideWindow) should suppress the flash without severing output.

Could you drop the DETACHED_PROCESS bit and confirm go test ./internal/tool/builtin stays green on Windows? Would also be good to add a proc-level test asserting child stdout is still captured with the flag set, since the internal/proc suite only checks the flag bits and missed this. Happy to merge once output capture is preserved.

Updated.

proc.HideWindow no longer sets DETACHED_PROCESS; it stays on CREATE_NO_WINDOW
so shell tools preserve stdout/stderr capture. I added a proc-level regression
test for child stdout capture and confirmed go test -count=1 ./internal/tool/builtin
passes on Windows.

The detached behavior is now scoped only to the desktop workspace git probes that
reproduce #2930, with a non-Windows no-op stub. I also kept CREATE_NO_WINDOW and
DETACHED_PROCESS out of the same shared helper path, matching the Windows process
creation flag semantics:
https://learn.microsoft.com/en-us/windows/win32/procthread/process-creation-flags