Frozen Region

The relative sizes of the frozen region (blue area), the screen (pink box).

The Frozen Region is a game mechanic wherein areas of the map that are far enough offscreen will become 'frozen'. Objects will not move or interact, and characters stop moving until you draw closer.

The frozen region starts a few tiles away from the edge of the screen, so enemies, bombs, and other items that go just off the edge of the screen don't stop moving until they get far enough away from the player, at which point they do completely freeze in place, and stay frozen until you move closer (see image).

Fortunately, this frozen region remains so far away and is so inconsequential that most of the time you won't even know it's in effect. Game Maker gives game authors enough tools to make it completely unnoticeable.
However, currently there are a few instances where it does affect gameplay. This behaviour can be considered a bug.

Examples include:

• Bullets fired will freeze at this border. If you have enough space, you can fire multiple shots with the pistol, and all shots fired will freeze at the same location. If you then move towards the bullets, all bullets will move as a unit. Using this method, you can kill high-hp enemies in "one shot". This allows the shopkeeper to be killed without risk. It is even possible to kill yourself with your own shotgun by moving around the frozen bullets at a distance.
• You can avoid the boulder trap completely by picking up the golden idol and then running away before the rumbling releases the boulder. The boulder will still appear once you get closer, but usually you're safely on a lower level by that point.
• When throwing an item a long way, it will still be falling (or bouncing) when you approach the place you threw it, even if you threw it a long time ago.

Note that there are exceptions where objects are 'immune' to freezing, such as the ghost, rolling boulders and the final boss, which can always move no matter where they are in relation to the screen.
As these entities work by 'unfreezing' a small bubble around themselves, objects they touch or pass over also become unfrozen briefly as well, even if they are normally affected by the frozen region.

Implementation

The frozen region is not a bug or a software limitation, but rather an intentional feature added to the game. The reason it exists is to optimise game performance - If the game had to constantly update every object on the entire level, the framerate would sometimes drop to unacceptable levels.
The frozen region is a standard feature available to all Game Maker games, and it's updated via the 'Deactivate instance' and 'Reactivate Instance' function calls.