In method nsCSSFrameConstructor::ConstructTextFrame,a TextFrame is
created for the token 'world'.
Lets's look at the structure from the TextFrame for 'world' up to the
root fame,its parent is a BlockFrame ,and the parent of the BlockFrame
is a nsAreaFrame ,then a CanvasFrame,a ScrollFrame,a ScrollFrame,finaly
the root frame is a ViewportFrame.
I don't know what these Frames are used for. Can anybody explain it for
me? By the way, does this part of the frame tree(from the TextFrame up
to the root frame) construct right?