discourse/app/assets/stylesheets/mobile/select-kit/toolbar-popup-menu-options.scss

// ensure popupmenu options has scroll behavior on limited screen space
#reply-control {
  &.composer-action-createTopic {
    .toolbar-popup-menu-options .select-kit-collection {
      // fits popup menu inside textarea in create topic composer
      max-height: 155px;
    }
  }

  &.composer-action-reply {
    .toolbar-popup-menu-options .select-kit-collection {
      // fits popup menu to textarea in reply composer
      max-height: 230px;
    }
  }
}