only capture enter if something was selected in completions

src/components/views/rooms/Autocomplete.js

@@ -162,6 +162,10 @@ export default class Autocomplete extends React.Component {
         });
     }
 
+    hasSelection(): bool {
+        return this.countCompletions() > 0 && this.state.selectionOffset !== 0;
+    }
+
     countCompletions(): number {
         return this.state.completionList.length;
     }

src/components/views/rooms/MessageComposerInput.js

@@ -989,7 +989,7 @@ export default class MessageComposerInput extends React.Component {
             return change.insertText('\n');
         }
 
-        if (this.autocomplete.countCompletions() > 0) {
+        if (this.autocomplete.hasSelection()) {
             this.autocomplete.hide();
             ev.preventDefault();
             return true;