Adding a stylesheet to the tree should be a synchronous operation with respect to content->layout queries

[jdm]

Before the introduction of LayoutRPC in #3164, this worked fine since messages are synchronous and ordered. However, now introducing a new stylesheet into the tree and immediately querying the layout properties of a node that is affected by the new sheet will yield incorrect information. I guess the code that notifies the layout task about the stylesheet should also cause a layout flush to occur, which should prevent the content->layout queries from executing until the new styles have taken effect.

[metajack]

When you say flush, do you mean reflow or just invalidation of the layout data? It seems like we should delay reflow until it is needed.

[jdm]

Well, I really mean reflow because our layout queries all rely on the display list. There's a FIXME about that.