1

Whenever an expression is too long, it will be broken automatically in inline mode but done nothing in displaymode

E.g.

MWE:

\documentclass[11pt]{article}
\begin{document}

$1+2+3+4+5+6+7+8+9+0+1+2+3+4+5+6+7+8+9+0+1+2+3+4+5+6+7+8+9+0+1+2+3+4+5+6+7+8+9+0+1+2+3+4+5+6+7+8+9+0$

\[1+2+3+4+5+6+7+8+9+0+1+2+3+4+5+6+7+8+9+0+1+2+3+4+5+6+7+8+9+0+1+2+3+4+5+6+7+8+9+0+1+2+3+4+5+6+7+8+9+0\]

 \end{document}

The upper one will be broken whereas the lower one not.

So how to make auto break also works in displaymode?