How do I prevent text overflow?

Truncate

The Truncate option under Text Fit allows you to shorten your text by creating a cutoff point, indicated by an ellipses (…).   

This should be used in conjunction with the Line Clamp to limit your text by a specific amount of lines.

Height

When you create a new text layer, the height of the text will be set to auto, which may cause text-overflow.

To prevent this, you can set the text container's Height by manually entering the height you want or dragging to resize the container.