fix: use correct wt.exe split-pane flags (-H/-V) instead of invalid --direction

internal/platform/coverage_test.go

@@ -155,6 +155,57 @@ func TestLaunchStyleConstants(t *testing.T) {
 	}
 }
 
+// ---------------------------------------------------------------------------
+// appendWTPaneDirFlags
+// ---------------------------------------------------------------------------
+
+func TestAppendWTPaneDirFlags(t *testing.T) {
+	tests := []struct {
+		name string
+		dir  string
+		want []string
+	}{
+		{"down", "down", []string{"-H"}},
+		{"up", "up", []string{"-H"}},
+		{"right", "right", []string{"-V"}},
+		{"left", "left", []string{"-V"}},
+		{"auto", "auto", nil},
+		{"empty", "", nil},
+		{"unknown", "diagonal", nil},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			base := []string{"-w", "0", "split-pane"}
+			got := appendWTPaneDirFlags(base, tc.dir)
+
+			// base should never be modified in place.
+			extra := got[len(base):]
+			if tc.want == nil {
+				if len(extra) != 0 {
+					t.Errorf("dir=%q: got extra args %v, want none", tc.dir, extra)
+				}
+				return
+			}
+			if len(extra) != len(tc.want) {
+				t.Fatalf("dir=%q: got %v extra args, want %v", tc.dir, extra, tc.want)
+			}
+			for i, w := range tc.want {
+				if extra[i] != w {
+					t.Errorf("dir=%q: extra[%d]=%q, want %q", tc.dir, i, extra[i], w)
+				}
+			}
+		})
+	}
+}
+
+func TestAppendWTPaneDirFlags_PreservesExistingArgs(t *testing.T) {
+	base := []string{"existing", "args"}
+	got := appendWTPaneDirFlags(base, "down")
+	if len(got) != 3 || got[0] != "existing" || got[1] != "args" || got[2] != "-H" {
+		t.Errorf("got %v, want [existing args -H]", got)
+	}
+}
+
 // ---------------------------------------------------------------------------
 // TerminalInfo and ShellInfo
 // ---------------------------------------------------------------------------

internal/platform/shell.go

@@ -537,6 +537,22 @@ func defaultWindowsShell() ShellInfo {
 	return ShellInfo{Name: "Command Prompt", Path: p}
 }
 
+// appendWTPaneDirFlags translates a dispatch pane direction into the correct
+// wt.exe split-pane flags (-H for horizontal/down, -V for vertical/right).
+// Windows Terminal does not support "up" or "left" natively, so those fall
+// back to -H and -V respectively (the closest available direction).
+func appendWTPaneDirFlags(args []string, dir string) []string {
+	switch dir {
+	case "down", "up":
+		return append(args, "-H")
+	case "right", "left":
+		return append(args, "-V")
+	default:
+		// "auto" or empty — let Windows Terminal choose.
+		return args
+	}
+}
+
 func launchWindowsSession(shell ShellInfo, resumeCmd string, terminal string, cwd string, launchStyle string, paneDirection string) error {
 	// Use Windows Terminal when configured (or defaulted by LaunchSession).
 	if terminal == termWindowsTerminal {
@@ -549,9 +565,7 @@ func launchWindowsSession(shell ShellInfo, resumeCmd string, terminal string, cw
 			case LaunchStylePane:
 				// Open a split pane in the current tab.
 				args = append(args, "-w", "0", "split-pane")
-				if paneDirection != "" && paneDirection != "auto" {
-					args = append(args, "--direction", paneDirection)
-				}
+				args = appendWTPaneDirFlags(args, paneDirection)
 			default:
 				// Open a new tab in the most recently used window.
 				// Without -w 0, wt.exe opens a new window by default.
@@ -879,9 +893,7 @@ func buildWSLWTArgs(shell ShellInfo, resumeCmd, winCwd, distro, launchStyle, pan
 		args = append(args, "-w", "new", "new-tab")
 	case LaunchStylePane:
 		args = append(args, "-w", "0", "split-pane")
-		if paneDirection != "" && paneDirection != "auto" {
-			args = append(args, "--direction", paneDirection)
-		}
+		args = appendWTPaneDirFlags(args, paneDirection)
 	default:
 		args = append(args, "-w", "0", "new-tab")
 	}

internal/platform/wsl_test.go

@@ -454,8 +454,11 @@ func TestBuildWSLWTArgs_LaunchStyles(t *testing.T) {
 		{"explicit_tab", LaunchStyleTab, "", []string{"-w", "0", "new-tab"}},
 		{"window", LaunchStyleWindow, "", []string{"-w", "new", "new-tab"}},
 		{"pane_auto", LaunchStylePane, "auto", []string{"-w", "0", "split-pane"}},
-		{"pane_right", LaunchStylePane, "right", []string{"-w", "0", "split-pane", "--direction", "right"}},
-		{"pane_down", LaunchStylePane, "down", []string{"-w", "0", "split-pane", "--direction", "down"}},
+		{"pane_right", LaunchStylePane, "right", []string{"-w", "0", "split-pane", "-V"}},
+		{"pane_down", LaunchStylePane, "down", []string{"-w", "0", "split-pane", "-H"}},
+		{"pane_left", LaunchStylePane, "left", []string{"-w", "0", "split-pane", "-V"}},
+		{"pane_up", LaunchStylePane, "up", []string{"-w", "0", "split-pane", "-H"}},
+		{"pane_empty_dir", LaunchStylePane, "", []string{"-w", "0", "split-pane"}},
 	}
 
 	for _, tc := range tests {