The composer is displayed over the bottom part of the page. To make sure
that no content is covered by the composer, a bottom padding is added
equal to the height of the composer. When the composer is opened or
closed that padding is added after around 300ms because of a debounce.
This commit makes sure that the padding is added as soon as the composer
state changes by using a CSS custom property (variable) and transition
property for a smooth user interface.
9ea8a4a9af