Make a container and put some text in it:
then add a negative margin to the heading, so that it become outdented:
no surprises so far:
Now add an overflow-y: auto to the container so that its content could scroll down. The moment you do this, the hanging part of the heading gets cropped by the border of the container:
Overflow-x: visible doesn't help. There must be a spec somewhere that explains why this is the correct behavior; i.e. why overflow-x starts behaving as if it were set to hidden; but it's so unintuitive it's bewildering.
(example in jsfiddle)
Of course the desired effect can still be achieved through a combination of padding and negative margin, but still — yikes!