From atotic@'s test case, I'm filing this issue because this needs 3 different expertise; inline, abspos, and margin collapsing.

With this CSS:
.rel { position: relative; }
.abs { position: absolute; }

The following HTML:

  <div><span class="rel"><span class="abs"></span></div>

is a "certain zero-height line boxes" that allows margin collapsing[1]. For LayoutNG, this means that BFC offset is not resolved for this <div>, but it needs a static position. Also, getBoundingClientRect() must return something for the <span class="rel">.

Test: the first 4 are empty line boxes, the last 2 are non-empty.
http://jsbin.com/kilibix/edit?html,output

Options:
A. Make this line box not empty. Easiest for the current architecture, but no browsers do this.
B. Give (0, 0) for the static position. Gecko does this.
C. Give the top of the next BFC offset. Blink/Edge/WebKit do this.

C looks most reasonable, but this looks like a challenge to our BFC/margin collapsing code.

Ian, any idea how to do this, or do we prefer other options?

[1] https://drafts.csswg.org/css2/box.html#x28