Should "adjacent spanners" be defined in more detail? Here's what we have today:

https://drafts.csswg.org/css-multicol/#spanning-columns

Spanners are block-level boxes therefore the margins of two adjacent spanners will collapse with each other. As column boxes establish a new block formatting context, margins on elements inside a column box will not collapse with the margin of a spanner.

So it looks like a column box between two spanners will prevent them from being adjacent, but when exactly do we create column boxes?

<div style="columns:3; width:100px; background:blue;">
  <div id="s1" style="column-span:all; margin-bottom:50px; height:25px;"></div>
  <div id="abs" style="position:absolute;"></div>
  <div id="s2" style="column-span:all; margin-top:50px; height:25px;"></div>
</div>

Are #s1 and #s2 adjacent spanners? Not if #abs causes a column box to be created, for sure, but should it? If it shouldn't, should we still treat #s1 and #s2 as adjacent?

Browsers don't agree. Blink and EdgeHTML draw a 100x100 square (i.e. adjacent). Gecko draws a 100x150 rectangle (i.e. not adjacent). Currently the new fragmentation engine that we're working on in Blink behaves like Gecko, but this is merely an outcome of implementation details, and not a conscious choice.