The commandbar is defined with a width of 100% and a padding of .5em. Browsers as Firefox and Chrome use a box-sizing model "content-box" as default. In this model padding and border are added to the defined width of the element, which results in a width of over 100% and (depending on the theme) will lead to ugly horizontal scrollbars.
I would suggest adding the following lines to the rule
This forces a display with the box model "border-box". In this model the width includes padding and border.