In short: the DSP part should write an Atom event to notify the GUI.

The long: Ardour3 deletes UI instances when the window is closed: that means the state of the UI is lost.

When re-opening the UI, it must re-request the state from the DSP part.

Cleanest way:

-On UI init(), write a "requestSamplerate" Atom message to the DSP

-On DSP recieve "requestSamplerate", write Atom message with samplerate int to UI.

That covers all the use cases, and the UI will auto update upon re-init in Ardour :)

HTH, -Harry