fix(desktop): stop console-window flashing on Windows (#399)

On Windows the daemon shells out to console-subsystem processes without
suppressing the console window. The reaper polls session liveness on a
timer (and the attach loop retries), each call runs a zellij command, and
every invocation pops a console window that instantly closes, so the user
sees rapid blinking.

- Add a platform-split hideWindow(cmd) helper to the zellij package
  (CREATE_NO_WINDOW + HideWindow on Windows, no-op elsewhere) and call it
  in execRunner.Run so list-sessions and friends stop flashing.
- startBackgroundProcess now uses CREATE_NO_WINDOW instead of
  CREATE_NEW_CONSOLE (the latter creates a console then hides it, flashing).
- Pass windowsHide: true to the daemon spawn in main.ts so the daemon's own
  console stays hidden on a Windows GUI launch.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

backend/internal/adapters/runtime/zellij/process_other.go

@@ -2,7 +2,13 @@
 
 package zellij
 
-import "errors"
+import (
+	"errors"
+	"os/exec"
+)
+
+// hideWindow is a no-op off Windows: only Windows pops a console window.
+func hideWindow(*exec.Cmd) {}
 
 // startBackgroundProcess is a stub: the fire-and-forget path is only used by
 // the Windows zellij codepath. Non-Windows builds create sessions

backend/internal/adapters/runtime/zellij/process_windows.go

@@ -10,12 +10,23 @@ import (
 	"golang.org/x/sys/windows"
 )
 
+// hideWindow suppresses the console window for a console-subsystem child so
+// frequent zellij calls (e.g. the reaper's list-sessions) don't flash on Windows.
+func hideWindow(cmd *exec.Cmd) {
+	cmd.SysProcAttr = &syscall.SysProcAttr{
+		HideWindow:    true,
+		CreationFlags: windows.CREATE_NO_WINDOW,
+	}
+}
+
 func startBackgroundProcess(env []string, name string, args ...string) error {
 	script := "Start-Process -FilePath " + psQuote(name) + " -ArgumentList " + psQuote(windowsCommandLine(args)) + " -WindowStyle Hidden"
 	cmd := exec.Command("powershell.exe", "-NoLogo", "-NoProfile", "-EncodedCommand", powerShellEncodedCommand(script))
 	cmd.Env = env
 	cmd.SysProcAttr = &syscall.SysProcAttr{
-		CreationFlags: windows.CREATE_NEW_CONSOLE,
+		// CREATE_NO_WINDOW, not CREATE_NEW_CONSOLE: the latter creates a console
+		// then hides it, which flashed a window.
+		CreationFlags: windows.CREATE_NO_WINDOW,
 		HideWindow:    true,
 	}
 	if err := cmd.Start(); err != nil {

backend/internal/adapters/runtime/zellij/zellij.go

@@ -93,6 +93,7 @@ type execRunner struct{}
 func (execRunner) Run(ctx context.Context, env []string, name string, args ...string) ([]byte, error) {
 	cmd := exec.CommandContext(ctx, name, args...)
 	cmd.Env = zellijCommandEnv(os.Environ(), env)
+	hideWindow(cmd)
 	return cmd.CombinedOutput()
 }
 

frontend/src/main.ts

@@ -506,6 +506,8 @@ async function startDaemonInner(startEpoch: number): Promise<DaemonStatus> {
 		env: daemonEnv(),
 		shell: launch.shell,
 		detached: true,
+		// Hide the daemon's console on a Windows GUI launch (no flashing terminal).
+		windowsHide: true,
 	});
 	daemonProcess = child;