Some suggestions about dhtmlxWindow.
The user should be able to adjust the height of the header via the init function.
1.1. The user should be able to customize the close, expand and park buttons with his own icons.
1.2. The user should be able to set the background color of the header and the container representing the window.
1.3. The user should be able to adjust the width of the left and right part of the container.
1.4. The user should be able to absolute position the (close, park, minimize) buttons outside of the header container.
1.5 The user should be able to put the window title in the center of the header.
Compact version of the window.
When the user is attaching a toolbar/tabbar there should be an option to choose if the toolbar/tabbar should be located on the header or the way it is now.
I understand that in this case the dragging may be challenging so maybe an absolute positioned small circle outside of the header may be used for dragging. The choice is yours and not a big deal for me.
Here are examples.