The offsetWidth/offsetHeight comment seems misplaced now, doesn't it?

Also, a comment about why the fallback is needed (below, at line 138) would be useful, it's not clear now.

I agree with @mgol on comment placement, and also observe that

• defining a new variable here is against our general style, and
• val is an overly generic name, and
• it provides little benefit anyway, since delta will never be modified when offsetWidth/offsetHeight is unknown (computedVal - computedVal - delta - extra - 0.5 is guaranteed to be negative in this case, so Math.ceil will not be positive, so Math.max will be 0).

I think we should keep things simpler:

// offsetWidth/offsetHeight is a rounded sum of content, padding, scroll gutter, and border
// Scroll gutter is that value minus the other properties, rounded half-down to the nearest integer
delta += Math.max( 0, Math.ceil(
	elem[ "offset" + dimension[ 0 ].toUpperCase() + dimension.slice( 1 ) ] -
	computedVal -
	delta -
	extra -
	0.5

// If offsetWidth/offsetHeight is unknown, then we can't determine content-box scroll gutter
// Use an explicit zero to avoid NaN (gh-3964)
) ) || 0;