Only one document or tool can have focus at a time, even when multiple
IDocumentDock instances or floating windows are present. The
IRootDock that is currently active exposes its focused dockable via the
FocusedDockable property. You can subscribe to the
IFactory.FocusedDockableChanged event or query the property directly.
factory.FocusedDockableChanged += (_, e) =>
Console.WriteLine($"Current document: {e.Dockable?.Title}");
var current = factory.GetCurrentDocument();
The helper GetCurrentDocument extension returns the focused document
for the active root dock or null if no document is selected.