W3C home > Mailing lists > Public > www-style@w3.org > February 2015

Re: [css-flexbox] Behaviour of percentage heights in column direction

From: Daniel Holbert <dholbert@mozilla.com>
Date: Mon, 09 Feb 2015 18:38:38 -0800
Message-ID: <54D96F2E.60500@mozilla.com>
To: Peter Salas <psalas@microsoft.com>, "Tab Atkins Jr." <jackalmage@gmail.com>
CC: www-style list <www-style@w3.org>
On 02/09/2015 04:14 PM, Peter Salas wrote:
> Instead of trying to carve out cases in these new algorithms where the height doesn't depend at all on the content, we could carve out the cases where it only depends on the content.  In other words, I think that ignoring percentage heights should be the exception, rather than the rule.

If we were going in that direction, I'm not sure I understand the point
of singling out cases where the height *only* depends on the content.
Why stop there? e.g. if we have a scenario where e.g. a container's size
depends on its contents plus one other factor, and you say we should
allow percent-resolution, then I don't see why we'd change our behavior
to block percent-resolution when that other factor goes away (or shrink
its influence to 0px via a transition or something).

Anyway -- as I noted in "SUGGESTION" else-thread[1] (the latter part of
that message), I think I'm currently leaning in the opposite direction
-- reducing the cases where percent resolution is allowed -- for
performance & consistency reasons.

I suspect (?) the traditional CSS percent-handling behavior (treating %
values as 'auto' if the container depends on their size) is partly
intended to avoid the need for multi-pass layout, and the resulting
exponential blowup in deeply-nested situations.  Whenever a container
has to layout its children twice (once for measuring, once to let them
resolve their final percent sizes), there's a potential for painful perf
effects when things are deeply nested.  Even if this wasn't an explicit
goal when CSS percent-handling was being designed, it's at least a happy
side-effect of the rules, and it's an important thing to keep in mind
when thinking about tweaking things & how to apply this to new layout modes.

~Daniel

[1] https://lists.w3.org/Archives/Public/www-style/2015Feb/0213.html
Received on Tuesday, 10 February 2015 02:39:09 UTC

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