Bug 70795 – Need to figure out how flexboxes interact with column breaks, page breaks, etc.

• Status: NEW
• Component: Layout and Rendering

[Ojan Vafai]

http://dev.w3.org/csswg/css3-flexbox/#pagination

[Ojan Vafai]

12:24 PM <TabAtkins> ojan: Yes, there's an entire section in the spec marked as "non-normative" detailing exactly how we think flexbox pagination should work.  It's non-normative because we want feedback on it from implementors.
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12:30 PM <TabAtkins> If the answer ends up being "they're atomic for breaking purposes", that would be sad but acceptable.
12:30 PM <TabAtkins> ojan: Though, what happens when you try to print a webpage laid out using flexbox?
12:30 PM <dhyatt> that's my concern. i doubt anyone even knows. :)
12:30 PM <dhyatt> probably something terrible
12:31 PM <ojan> dhyatt: does CSS pagination and printing use the same logic?
12:34 PM <dhyatt> ojan: all pagination is sharing breaking logic right now
12:34 PM <dhyatt> ojan: what i'm working on now is bringing columns/pages/regions painting and hit testing and that sort of thing even closer together also
12:34 PM <dhyatt> but for the purposes of a layout system like flex box or grid
12:35 PM <dhyatt> "paginating" you don't really have to think about regions/columns/pages
12:35 PM <dhyatt> you just care about "breaks"
12:35 PM <ojan> dhyatt: i see...making sure printing flexboxes works reasonably seems higher priority than CSS columns/pages IMO. i didn't realize it used the same logic.
12:36 PM <dhyatt> really the big thing to know
12:36 PM <dhyatt> is that you want to set your correct block progression position before laying out
12:36 PM <dhyatt> i.e., y position in horizontal text
12:36 PM <dhyatt> if you can't do that
12:36 PM <dhyatt> e.g., because flexing pushes you
12:36 PM <dhyatt> then you have to relayout
12:36 PM <dhyatt> because how you have to paginate may change
12:37 PM <dhyatt> flex boxes are probably very difficult to paginate (like tables)
12:38 PM <dhyatt> well i don't really get how flexing works
12:38 PM <dhyatt> like do you lay out unflexed and paginate?
12:38 PM <dhyatt> and then flex and re-paginate?
12:38 PM <dhyatt> that could get weird
12:38 PM <TabAtkins> dhyatt: Not *quite*.
12:39 PM <TabAtkins> I forget details now, but it tries to be as close to normal flex layout as possible, without being expensive or crazy.
12:40 PM <dhyatt> ojan: well the big thing to know is just that there's a relayout you need to do if your position moves in such a way that pagination has to be redone
12:40 PM <dhyatt> that and try to set the correct position in advance
12:40 PM <dhyatt> since if you do, pagination will just work.
12:40 PM <TabAtkins> And it has substantially different behavior for the four cases of row/column and single-line/multi-line, with multi-line columns being the craziest.
12:41 PM <dhyatt> TabAtkins: well it's cool you wrote something down at least! :)
12:41 PM <ojan> taking a quick look at the spec, i think it tries to set the correct position in advance
12:41 PM <ojan> assuming i understand what you mean by that :)
12:41 PM <dhyatt> yeah i just mean e.g., block layout makes a y-estimation for example
12:42 PM <dhyatt> and tries to lay out at the correct y position
12:42 PM <dhyatt> if it ends up being wrong and has to move
12:42 PM <dhyatt> then it will relayout
12:42 PM <dhyatt> because being paginated requires that
12:42 PM <dhyatt> markForPaginationRelayoutIfNeeded
12:42 PM <dhyatt> is the function that handles it
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12:44 PM <dhyatt> ojan: i mean, the one thing i'd suggest is we just make sure it doesn't go toes up
12:44 PM <dhyatt> ojan: since crashes would be bad
12:44 PM <ojan> dhyatt: yeah, we should at least add tests showing that we are doing something sane or insane
12:44 PM <dhyatt> for unflexing flex boxes
12:44 PM <dhyatt> as long as you place them as you go
12:44 PM <dhyatt> they should paginate correctly
12:44 PM <ojan> dhyatt: although, i expect our fuzzer would catch most crashes in this area
12:45 PM <dhyatt> for flexing it depends on if you relayout if you flex
12:45 PM <dhyatt> if you do, then they might kinda work too