fix interaction of buffered/unbuffered output

Fixes #3747

src/reader.cpp

@@ -682,12 +682,16 @@ void reader_write_title(const wcstring &cmd, bool reset_cursor_position) {
         }
         fputwc(L'\a', stdout);
     }
+
     proc_pop_interactive();
     set_color(rgb_color_t::reset(), rgb_color_t::reset());
     if (reset_cursor_position && !lst.empty()) {
         // Put the cursor back at the beginning of the line (issue #2453).
         fputwc(L'\r', stdout);
     }
+
+    // TODO: This should be removed when issue #3748 is fixed.
+    fflush(stdout);
 }
 
 /// Reexecute the prompt command. The output is inserted into data->prompt_buff.