Only the default slot of <wa-page> should scroll horizontally

[fmercille]

Using 3.0.0-alpha.9

Describe the bug

When using <wa-page> as the main layout of a page, and the default slot contains something that overflows horizontally, the entire page, including headers, footers and the navigation panel will scroll horizontally. Only the default slot should scroll so that the other slots remain visible and "sticky" on the horizontal axis.

To Reproduce

Steps to reproduce the behavior:

1. Create a page using <wa-page> and populate the slots for header, navigation
2. In the default slot, display something that is wider than the viewport (e.g. a <table> with dozens of columns)
3. Scroll the page horizontally (resize your browser window if necessary)
4. Notice that the navigation scrolls away, as well as the header.

Demo

Here's a codepen showing the issue: https://codepen.io/fmercille/pen/ogvQXLX

Load it, and then scroll the page horizontally. Notice how the banner, header, subheader, main-footer and footer do not extend all the way (which is less than ideal if they are themed according to the brand of the site), and that the navigation panel disappears.