Determining when line breaks should occur on small screens in 4.0.0 with firefox.

In testing if some of my formulas are more responsive using the line break feature of 4.0.0, I have determined that sometimes line breaks occur on smaller screens and other times they don't. I haven't figured out the logic here and if it is something that I can improve or configure.

I have an equal chain that doesn't line break on smaller screens.

\(A\vec{\bf x} =
 \begin{bmatrix} 2 & 2 \\ -1 & 3\end{bmatrix}
 \begin{bmatrix} 2 \\ 3\end{bmatrix}
 = \begin{bmatrix} (2)(2) + (2)(3) \\ (-1)(2) + (3)(3) \end{bmatrix}
 = \begin{bmatrix} 10 \\ 7 \end{bmatrix}\)

But if I add another matrix to the equal chain it does line break.

\(A\vec{\bf x} =
 \begin{bmatrix} 2 & 2 \\ -1 & 3\end{bmatrix}
 \begin{bmatrix} 2 \\ 3\end{bmatrix}
 = \begin{bmatrix} (2)(2) + (2)(3) \\ (-1)(2) + (3)(3) \end{bmatrix}
 = \begin{bmatrix} 4 + 6 \\ -2 + 9 \end{bmatrix}
 = \begin{bmatrix} 10 \\ 7 \end{bmatrix}\)

Here is a screenshot, the first equal chain the last matrix is pushed off the edge of the screen, while the second causes a line break.

In trying to determine what causes the line breaks, it seems to be the number of matrices,
and not their sizes. For example if I make the last matrix bigger in the first example by writing
the numbers as a sum of 1's:

\(A\vec{\bf x} =
 \begin{bmatrix} 2 & 2 \\ -1 & 3\end{bmatrix}
 \begin{bmatrix} 2 \\ 3\end{bmatrix}
 = \begin{bmatrix} (2)(2) + (2)(3) \\ (-1)(2) + (3)(3) \end{bmatrix}
 = \begin{bmatrix} 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1 + 1\\ 1 + 1 + 1 + 1 + 1 + 1 + 1 \end{bmatrix}\)

No line break occurs just like the first example, even though this statement is now longer than
the second example which has a line break in it.

I did notice there have been various issues and some fixes with line breaks in 4.0.0, but I could not determine if this was addressed (this particular issue didn't seem to be included in the other issue and fixes I saw, and I didn't have time to test if this was fixed by building the most recent mathjax from source).