Shapes: cannot get 100% width

I'm trying to create a "header" region in my course presentation content by creating a rectangle shape with a white background.

When I resize shape and attempt to make it stetch the whole width of the presentation canvas, I am still left with a gap on the right-side of the shape:

When I inspect the shape element, I can see that container has a max width of 99.7821%:

If I change this to width: 100%, the gap is no longer present.

Note: In the H5P editor, the max width I can set is 918.

Any help would be appreciated.