The start title buttons of the main window should be displayed if and
only if the end title buttons are. Only the left-most header bar should have
the start title button set and only right-most header bar the end title buttons.
Therefore, the title buttons property should be bound to the leaflet state:
When unfolded the start title buttons of the sidebar's header bar are
the only ones visible then, and not a second pair in the panel's header bar.
Likewise, in a folded state of the leaflet, the panel's header bar displays
all availble title buttons – start and end – as the panel is the only widget
displaying a header bar.
Signed-off-by: Markus Göllnitz <camelcasenick@bewares.it>
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