49938 - removing empty line boxes

Status: RESOLVED WORKSFORME

(Core :: Layout: Block and Inline, defect, P3)

Description

[David Baron :dbaron: (⌚️UTC-5, no longer working on Mozilla)]

We could probably save a bit of memory (not that much, though) by removing empty
lineboxes that have only whitespace.  When we have something like this:

<div>
  <div></div>
</div>

the outer div has three line boxes:  one for the initial whitespace, one for the
inner div, and one for the final whitespace.  We have no need for the first and
third line boxes and the text frames they contain.

Doing this would allow us to simplify a bit of code in layout (although we still
have to be careful of line boxes with only placeholder frames).

Perhaps it could be done on a pass at the end of frame construction.  We would
only want to do this if 'white-space' were not 'pre' or '-moz-pre-wrap', so we
would have to make the whitespace property reframe, rather than just reflow (as
I think it does now, although I'd have to check).

I'm not sure if this is worth the gain for the first release, but it might be
good to do sometime, so I'm filing a bug.

Comment 1

[buster]

good idea, but not a big enough win to justify working on for the first release

Comment 2

[Kevin McCluskey (gone)]

Build reassigning Buster's bugs to Marc.

Comment 3

[David Baron :dbaron: (⌚️UTC-5, no longer working on Mozilla)]

Taking, although I think I have another bug on this somewhere (filed by rbs?)
with some work.

Comment 4

[Mats Palmgren (inactive)]

The frame tree I get for the example in comment 0 is:

Block(div)(0)@7fc8dfc51b30 {0,0,35640,0} <
  line 7fc8dfc51c90: count=1 <
    Block(div)(1)@7fc8dfc51be0 {0,0,35640,0} <
    >
  >
>