Here is current Firefox UI layout for 95.x: https://i.imgur.com/fDeWGc4.png I'm trying to understand what causes it to expand even when its child `toolbar#nav-var` is set to be positioned absolutely. It no longer should contribute to the parent height or layout yet it still does. My frist thought was it is JS script that calculate the height and sets it however, there are no direct in-line height changes at all. Here is the test css: toolbox#navigator-toolbox > toolbar#nav-bar { position: absolute!important; height: 0!important; min-height: 0!important; } toolbox#navigator-toolbox > toolbar#nav-bar:focus-within, toolbox#navigator-toolbox > toolbar#nav-bar:hover, toolbox#navigator-toolbox > toolbar#nav-bar:focus { position: absolute!important; height: inherit!important; min-height: inherit!important; } Using CTRL+L shotrcut combination you can force the nav-bar inputbox to be `focused` which sets custom style that reveals it. Since it is positioned absolutely it should not contribute to layout yet when it appears, the render viewport shifts and `toolbox#navigator-toolbox` height automatically increases.