The new Horizon theme has a bug where app blocks added to the Header are not able to expand to the full width, even if the block settings are set to Full width. This was tested on the latest Horizon 1.0.1.
Here are some steps to reproduce this:
Add an app block that expands to full width to the Header:
Set the width to Full under the block settings:
The app block will not expand to the full width:
As a temporary workaround, you can use this Custom CSS on the block to allow it to expand to full width:
.shopify-block {
width: 100%;
}