Backspace deletes initial contents

[NSoiffer]

Description

If the editor starts with initial contents, the entire contents will disappear with a single backspace key... sometimes.

Steps to Reproduce

1. Start with mathlive/examples/basic/index.html. This begins with initial contents f(x)=
2. Click after the = and type a. Good.
3. Type a backspace. The a goes away. Good.
4. Type backspace again. The entire contents goes away. Bad.

Note that if I begin by typing a backspace, then the = goes away as do each of the characters, one-by-one, with each backspace as I would expect.