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[css3-flexbox] resolving flexible lengths

From: Ojan Vafai <ojan@chromium.org>
Date: Thu, 29 Sep 2011 20:39:07 -0700
Message-ID: <CANMdWTu8=b+8ftPENPCfcNpGrL8DP8FKT-mXQyuYRocRJ3BGBg@mail.gmail.com>
To: www-style@w3.org
There are a couple issues with the
http://dev.w3.org/csswg/css3-flexbox/#resolving-flexible-lengths step.

1. It's not clear that the preferred size ignores min/max constraints. I
think the spec should explicitly state that it ignores min/max constraints
unless the preferred size is set to auto, in which case it takes them into
account.

The use-case this allows is the following:
<div style="display: flexbox; width: 200px">
    <div style="width: flex(1); min-width: 100px;"></div>
    <div style="width: flex(3);"></div>
</div>

Clearly you would expect the first flex item to be 100px wide and the second
to be 300px. If the preferred width took min-width into account you would
instead get 125px and 75px.

2. It should be a requirement that flex items with a positive flex fill the
flexbox if possible given min/max constraints. Specifically, the currently
specced algorithm does the wrong thing in this case:

<div style="display: flexbox; width: 200px">
    <div style="width: flex(1 0 50px); min-width: 100px;"></div>
    <div style="width: flex(1 0 100px); max-width: 50px;"></div>
</div>

The first flex-item should have a width of 150, but instead it currently
gets a width of 100. Talking to Tab offline he suggested the following
solution:
"Looks like the fix is to separate them into different steps, with either
one having the ability to restart the algorithm.  If there is
positive free space, resolve max first; if there is negative free space,
resolve min first."

Seems fine to me.

3. Finally, it's not clear to me whether this algorithm is stable.
Specifically, I believe it matters what order you iterate through the flex
items. If it does, then the spec should clearly state the order it uses. The
options I see are layout order and document order. I think the former is
probably more intuitive, although I believe layout order would need to take
direction (and writing-mode?) into account in addition to flex-flow and
flex-order.

Ojan
Received on Friday, 30 September 2011 03:40:00 GMT

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