Single operator ignores line breaks if only \mbox's are used

Created by: ArcadeMissile

Demo

For a small live test, go to the demo on CodePen.

Steps to reproduce

1. Use only \mbox's and line breaks (\\)
2. Add only one operator anywhere (tested using \times, \cdot, +, -, *, /)

Example

Using \[\mbox{A} + \mbox{B} \\ \mbox{C} \] gives the following:

Using another online latex editor gave the correct result:

Technical details:

• Happens on version 2.5, master branch (commit: ba9afeb5)
• Also happens on develop branch (commit: 930e35a3)
• Tested Operating Systems: Windows 10, Mac OS X 10.10.4
• Tested Browsers: Chrome (44.0.2403.125), Firefox (39.0)

Thanks.