You don't say what version of MathJax you are using. If you are not using a v4 (now in beta release), then MathJax's in-line expressions should not break. If they do, then there may be CSS on your page that is interfering with MathJax's output, so look for that first.
If you are using v4, you can turn off in-line breaking via the MathJax contextual menu (in the "Wide Expressions" submenu of the "Math Settings" menu). You can make that the default for all users by setting the output.linebreaks.inline option to false in the MathJax configuration:
MathJax = {
output: {
linebreaks: {
inline: false
}
}
};
Users will still be able to change that setting in the MathJax contextual menu. See the
release notes for more details.
Davide