/* to hide the native top tabs */
#TabsToolbar * {
visibility: collapse;
}
/* to give the top toolbar a bit of height so the window controls don't dangle off to the right */
#TabsToolbar:before {
content: '';
display: block;
width: 100%;
height: 20px;
}
/* to hide the sidebar header */
#sidebar-header {
visibility: collapse;
}
Have you tried the built-in titlebar setting in Customize? There's an option at the bottom to turn on the regular system titlebar so that it displays as a normal window.
