Yeah, it is worth investigating. I'll work on this.

Quick experiment shows 3 tests to fail:
* external/wpt/shadow-dom/MouseEvent-prototype-offsetX-offsetY.html
* fast/alignment/parse-alignment-of-root-elements.html
* shadow-dom/layout-1.html
https://codereview.chromium.org/2945993002

Maybe these read style/layout after UpdateDistribution() without ensuring layout clean? Haven't really looked into them, just wild guess.

I have not looked deeper, but ensuring layout clean is the engine's responsibility, instead of user's responsibility, when a user accesses some info which needs clean layout, isn't it?

Do you mean "engine" is lifecycle and "user" is our code using layout?

I'm not very familiar with lifecycles, so I might be wrong, but it looks to me that LazyReattachIfAttached() is not lifecycle friendly, since it destroys LayoutObject immediately.

Mark dirty and let the cycle to clean it up looks more lifecycle friendly, and hence this bug...did I misunderstand somewhere?

Ah, I meant *Blink* by engine, and JavaScript by users.
In other words, I thought that users don't have to call document.body.offsetLeft before calling some other APIs which need clean layout.

Basically, I strongly support to stop using LazyReattachIfAttached there.
My concern is a regression.

> Ah, I meant *Blink* by engine, and JavaScript by users.

Oh, I see now. In my previous comment:
> Maybe these read style/layout after UpdateDistribution() without ensuring layout clean?

By "these" I meant "our code in Blink used by these tests", not "these javascript". Sorry for to be too terse.

> Basically, I strongly support to stop using LazyReattachIfAttached there.
> My concern is a regression.

Agreed. And I guess you're the only person who can make it right.

I have taken look at a failing test, shadow-dom/layout-1.html.
This looks a real regression; a shadow host's child would be rendered even if a shadow root is attached.

It looks we need further tweaks for layout tree construction itself, as well as marking flag dirty. Let me take a deeper look.

Let 776656 unblock this. This is a more general issue.