It is working in the same way for subSilver 2 which is a default style for phpBB3.
You may try to open a ticket in the Bugs Tracker @ phpBB.com.
Otherwise you may try to add css overflow property to that text field.
.post-text-hide-flow {
overflow: auto;
max-width: 960px;
}
Something like this will work for most cases.
P.S.: By the way you should forbid users to post such kind of "non-words".