If you want it to all be on the same line, then the easiest / quickest way of doing it would be to find the breakpoint at which it splits and add a media query in your CSS file which reduces the font size of the title to fit anything below that breakpoint. For example:
@media screen and (max-width: 499px) {
.site-branding .site-title {
font-size: 1.4em;
margin: 6px 0 0;
}
}
That makes the text smaller and adds a top margin to balance out the title against the height of the menu button.
Be sure to check on the smaller screen sizes than the breakpoint you’ve set that it all looks ok.
Alternatively you could create a max-width in the same media query, this will keep the title text the same size, potentially splitting the title into two lines, but it’ll still look tidy:
@media screen and (max-width: 499px) {
.site-branding .site-title {
max-width: 249px;
}
}
They’re not perfect fixes, ideally you’d tweak the HTML too, but the problem may be fixed in a theme update.