This is more of a targeted fix towards your website, instead of a general fix:
(Paste the code below to: Admin > Theme Options > Custom CSS)
.qa-post-content {
overflow-x: auto !important;
}
If you are the plugin owner, then you could wrap all the equation elements inside of a parent element, like:
<div class="mjx-parent">
<mjx-container></mjx-container>
<mjx-container></mjx-container>
. . .
</div >
Then it would be easier to implement a general fix like this instead:
.mjx-parent {
overflow-x: auto;
}