Touchgram's split personality - the menu and the editor
An iMessage app appears initially in the space normally taken up by the keyboard, at the bottom of the screen. Apple call this the Compact View.
Some simple apps do all their work in that compact view, like picking stickers.
Richer apps like drawing apps and Touchgram use that space as a starting menu and do most of their work in the Expanded View.
At any time, you can drag that dividing bar or just tap it to switch between views. If you're in the middle of editing a Touchgram, your work is saved.
Note that saving currently only remembers the details after you have pressed Save in a detail editor. So if you're halfway through designing a sensor for touches and accidentally collapse back to the menu on the Compact view, you will lose that sensor.