patch 9.0.0597: cannot close a tab page with the middle mouse button

Problem:    Cannot close a tab page with the middle mouse button.
Solution:   Support closing a tab page with the middle mouse button, like many
            other programs. (closes #10746)

src/gui_gtk_x11.c

@@ -3345,6 +3345,12 @@ on_tabline_menu(GtkWidget *widget, GdkEvent *event)
 		send_tabline_event(x < 50 ? -1 : 0);
 	    }
 	}
+	else if (bevent->button == 2)
+	{
+	    if (clicked_page != 0)
+		// Middle mouse click on tabpage label closes that tab.
+		send_tabline_menu_event(clicked_page, TABLINE_MENU_CLOSE);
+	}
     }
 
     // We didn't handle the event.

src/gui_w32.c

@@ -8094,6 +8094,20 @@ tabline_wndproc(
 		}
 		break;
 	    }
+	case WM_MBUTTONUP:
+	    {
+		TCHITTESTINFO htinfo;
+
+		htinfo.pt.x = GET_X_LPARAM(lParam);
+		htinfo.pt.y = GET_Y_LPARAM(lParam);
+		idx0 = TabCtrl_HitTest(hwnd, &htinfo);
+		if (idx0 != -1)
+		{
+		    idx0 += 1;
+		    send_tabline_menu_event(idx0, TABLINE_MENU_CLOSE);
+		}
+		break;
+	    }
 	default:
 	    break;
     }

src/version.c

@@ -699,6 +699,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    597,
 /**/
     596,
 /**/