Sticky positioning tries to keep an element visible within the nearest
scrollport. However, the element can't be offset to go beyond its
containing block. We implement this as offset bounds.
The problem was that, if the element would already be overflowing its
containing block before applying the sticky positioning, then we were
forcing it to move inside the containing block. That was wrong, and is
solved by flooring or ceiling the offset bounds by zero.
Testing: Adding new tests
Signed-off-by: Oriol Brufau <obrufau@igalia.com>
Co-authored-by: Martin Robinson <mrobinson@igalia.com>
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