Q + L to deletes all data because of scratch's 5MB .json limit , without it the level and 3D objects data would be even 3 TIMES in the .json limit , first is in the script, seconds is in the lists, and third is in the preview list. To delete data from preview lists (lists GUI) you need to delete all data from that list, show it and hide it. In order to save your .json must be below 5MB , so you have to delete the data first , it's automated with keyboard shortcut. The choice of Q+L is because you there are moments you need to save the game scripts while you aren't in debug , so to avoid accidentally pressing L , it is made to be Q+L