Post Editor: iframe editor canvas padding is overridden by zero in the classic theme

[t-hamano]

Description

When a classic theme is active, the body element is expected to have 8px padding.

In WP 6.9:

However, this padding is unintentionally overridden by zero in WP 7.0 and Gutenberg:

This causes the text to stick to the edge of the canvas, which can make it look slightly unappealing and affect text editing.

I've identified that this issue was caused by #73971. Specifically, the zero-padding here became apparent because global styles were internally enabled in the classic theme.

Step-by-step reproduction instructions

1. Activate Twenty Twenty
2. Open the post editor
3. Narrow your browser width or switch to tablet view

[t-hamano]

This zero padding also affects full-width blocks. The block overflows because a negative margin of 8px is applied to the left and right, despite the canvas having no padding.