suite/themes/modern/communicator/sidebar/customize.css (3 lines of code) (raw):
/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
#other-panels {
min-width: 180px;
}