I would suggest starting with the widget inspector and turning on mouse events tracking for the events you’re interested in. This should get you the widget that the mouse thinks it is interacting with, and breakpoints can be set from there.
Depending on the widget, there may be some wildly different areas to put breakpoints, so determining the SWidget in the path is probably the best place to start.
FMouseDeltaTracker is where the viewport inputs should eventually go through for drag operations (your first case)
Window layout should eventually hit SDockTab::OnMouseButtonDown (your second case)
Mouse clicks generally operate off of mouse up events, not mouse down, so I suspect the culprit widget has an implementation for mouse down that is causing the drag operations not to start, but the mouse up to process as you expect.