Here's a version of John's script that will select the event immediately to the left of the cursor. Unlike John's script, it selects the event even if there is a gap after it (or it's the last event on the track), and only if it reaches to the cursor (i.e. following a split or an ALT+] or if the cursor has been snapped to the event's end).

/** 
* PURPOSE OF THIS SCRIPT:
*
* Select event PRIOR to the cursor, and de-select all other events.
*
* A video track must be selected. If an audio track is selected, nothing happens.
* If the track beneath the video track contains audio, the audio event in that track 
* prior to the cursor the cursor will also be selected.
*
* Copyright © John Meyer 2004
* Written: September 23, 2004
* Updated October 21, 2014
* Modified by Nick Hope, October 07, 2016, to select the event immediately prior to the cursor,
* even if there is a gap after it, as long as it reaches the cursor.
*
* https://www.vegascreativesoftware.info/us/forum/auto-select-left-clip-after-video-splitting--98852/
*
**/ 

import System;
import System.IO;
import System.Windows.Forms;
import Sony.Vegas; // For VP13 build 453 and earlier
//import ScriptPortal.Vegas; // For VP13 build 543 and later

try { 
	//Global declarations
	var dStart : Double;
	var dLength : Double;
	var dCursor : Double;
	var trackEnum : Enumerator;
	var evnt : TrackEvent;
	var CurrentEvnt : TrackEvent;

	var track = FindSelectedTrack(); // Use this function to find the first selected track.  
	var eventEnum = new Enumerator(track.Events);

	if (track.IsVideo()) { // Proceed only if selected track is video track.
		if ( SelectEventAtCursor() ) { // Function that selects events under cursor.

		// Get set to look at track directly below the video track.
		trackEnum.moveNext(); // Go to next track.

			if (!trackEnum.atEnd()) { // Only proceed if there is a track below the video track.
				track = Track(trackEnum.item());// When doing the first track (above), these two lines were executed
				eventEnum = new Enumerator(track.Events); // in the FindSelectedTrack() function.
				if (track.IsAudio()) {// Only trim the event if this is an audio track.
					SelectEventAtCursor();
				}    
			}
		}
	}
	//  Vegas.Cursor = CurrentEvnt.Start;  // Enable this line to move cursor to start of selected event.
	Vegas.UpdateUI(); 

} catch (e) { 
	MessageBox.Show(e); 
} 

// End of main program


// Beginning of functions

function FindSelectedTrack() : Track {
	trackEnum = new Enumerator(Vegas.Project.Tracks);
	while (!trackEnum.atEnd()) {
		var track : Track = Track(trackEnum.item());

		if (track.Selected) {
			return track;
		}
		trackEnum.moveNext();
	}
	return null;
}

/** 
*
* The following function finds the event on the selected track 
* that lies under the cursor. It also deselects all other events. 
*
**/

function SelectEventAtCursor() {
	dCursor = Vegas.Cursor.ToMilliseconds(); // Remember the cursor position.

	//Go through each event on the track.
	CurrentEvnt=TrackEvent(eventEnum.item());

	while (!eventEnum.atEnd()) {
		evnt = TrackEvent(eventEnum.item());
		evnt.Selected = false; // De-select the event

		// Get the event's start and length timecode, in milliseconds.
		dStart = evnt.Start.ToMilliseconds();
		dLength = evnt.Length.ToMilliseconds();

/**
*     If the cursor timecode is between the beginning and end of the 
*     event timecodes, then select the event.
**/ 

		if ( dCursor == (dStart + dLength) ) {
			evnt.Selected = true; // Select this event.
		}
		CurrentEvnt = evnt;
		eventEnum.moveNext(); // Go to next event on this timeline.
	} 
}

The right black button on my Contour Shuttle Pro 2 now does this 3-stage macro:

• Runs a script that deselects all events then does ALT+]
• Then a 100ms pause (so that the next bit works. Might need to be longer in some cases)
• Then runs the above script so I get immediate feedback on the trimmed event length in the Show Event Length extension.

I also have an equivalent ALT+[ macro on the Shuttle's left button, but the pause isn't required, and it uses the first script in this comment instead of the one above.

Slick workflow for fast cutting.