Indicates whether the panel is empty.
Indicates whether the panel is expanded, collapsed, or in a transition between the two.
The index of the last tab that was selected. When the panel is expanded, it tries to restore the tab selection to the previous state.
The width or height of the panel before it was collapsed. When the panel is expanded, it tries to restore its size to this value.
A promise that is resolved when the currently pending side panel updates are done.