I like your example and I have a couple of comments that some readers may find helpful.
In your .cap, it may be noticed that the player starts to fall when the "up" key is released but only while stuck to a wall on his left, (when the "up" key is released when the player is stuck to a wall on his right, the player does NOT go anywhere.)
I am not sure if that was intentional, but this can be fixed 2 different ways:
To make the player STUCK when the "up" key is released on BOTH walls:
a. Insert condition "has wall to left" in event 1
b. Insert condition "OR" in event 1
c. Delete events 4 and 5
Event 1 should look something like this:
1 sprite3 has wall to left
OR
sprite3 has wall to right
2 ......
In order to change things so that the player falls while stuck to either wall, KEEP my changes from above and add another SUB-EVENT to event 1 and most importantly remember to INVERT this new condition:
(inverted condition) Player 1 "move up" is down Sprite 3: set gravity to 1000