Is it possible to detect if a midi on event is halfway through a note - i.e. a user resumed playback halfway through a note, as opposed to just started playing the note?

I want to resume playback of my audio - rather than starting fresh playback

You would need to keep track of which notes have received a noteOn message with no corresponding noteOff. MidiKeyboardState does this — just send it all the incoming MIDI data with processNextMidiBuffer then call isNoteOn to check if a note is already playing.

However this only works if playback was stopped and started in the same spot, without seeking. MIDI is inherently stateful, so there is no way to check if a note is on without keeping track of noteOn and noteOff events.

This won’t work in all DAWs, because some DAWs will send all note offs (or an all sound off events) when playback stops.