That's an interesting find and sounds really painful.
Some ideas how Scirra could fix it:
- using uint16 arrays for the tiles, halving memory usage
- only store a specific amount of tiles larger than the visible size (for example 128 tiles wider and 128 tiles higher than the visible size, making sure this can't get out of hand)
- only keep the last few layouts in memory, instead of all tabs. This may also fix some other editor memory and stability issues