I don't know offhand what type="chrome" vs. type="content" trigger, but I suspect the blank lines are happening due to some interaction with the compositor. (I can imagine that maybe the chrome vs. content type attribute could impact what sorts of inputs we've got for the compositor and/or how it operates, but that's just a guess -- I truly don't know what those attributes do.)
RE the compositor -- to illustrate my guess, here's a profile I recorded when following the STR, with a screenshots track at the top: https://perfht.ml/2T9WMen
If you hover the screenshots track and move your cursor from before to after a composite operation, you can see that the compositor-thread's bursts are the moments where the blank lines appear/disappear. I'd suggest asking someone on the graphics team about what's responsible for that blanking.
(Side note: the long reflow times here are pretty horrible and should be spun off to another bug, if they're not already tracked in a bug... They seem bad regardless of whether the type is chrome or content. i.e. they're pretty long in both profiles from comment 0.)