Part boundaries…
If your situation is that you have several Parts on a MIDI track, and, for example, one of them has notes that occur beyond the Part start or end boundaries, these notes will normally be visible, unless you do…
Preferences>Scores>Editing>“Unlock Layouts when editing single parts”.
One caveat with this, however… If you have already done some Layout editing (e.g. spacing of bars etc.) on the staff containing these MIDI Parts, you will lose the Layout editing when you change the status of that Preference.