Cursor position jumps to start on backspace after completion

[NSoiffer]

Description

The insertion cursor jumps to the start of an expression after deleting some latex substitutions. I might have seen this for other substitutions, but this is a reproducible case to work on.

Steps to Reproduce

1. Open mathlive/examples/basic/index.html and click after the =
2. Type \alpha followed by a space. Great -- the Greek letter appears and insertion point is good.
3. Type a backspace. The alpha goes away but the insertion point is at the start of the line as shown below:

[arnog]

Fixed with 46c9140