BP Better Messages breaks the primary search bar widget in my theme (HelpGuru theme).
The search bar uses an AJAX element to show live search results, and that element has the ‘.ui-autocomplete’ class.
The conflict comes from this portion of bp-messages.css:
width: 99% !important; /* <----- This line */
I can’t find where BP Better Messages uses .ui-autocomplete, but if you add a more specific selector or find a way to avoid using ‘!important’, it should stop interfering with other themes/plugins.