[Bug 16265] New: Define transition for "z-index: auto"

https://www.w3.org/Bugs/Public/show_bug.cgi?id=16265

           Summary: Define transition for "z-index: auto"
           Product: CSS
           Version: unspecified
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: Transitions
        AssignedTo: dino@apple.com
        ReportedBy: ayg@aryeh.name
         QAContact: public-css-bugzilla@w3.org
                CC: cmarrin@apple.com, eoconnor@apple.com, smfr@me.com
            Blocks: 15844


The spec says z-index is transitioned as an integer.  So what happens if it's
"auto"?  Testing seems to indicate that IE10 Developer Preview and Firefox
13.0a1 jump straight to auto without trying to interpolate.  Chrome 19 dev and
Opera Next 12.00 alpha both transition to 0.  When the value would round down
to 0, Opera seems to leave it at 0 until the transition is done, when it
switches to auto.  Chrome seems to leave it at 1.  The Chrome/Opera behavior
makes more sense, and I suggest we spec something like it.  E.g.:

1) If the transition is from 0 to auto or vice versa, it jumps directly with no
transition, same as properties with discrete values.

2) If the transition is from auto to a nonzero number, the computed value
immediately becomes 0, then transitions from there to the new value.

3) If the transition is from a nonzero number to auto, the computed value
transitions to 0, then becomes auto after the transition is done.

The details of when exactly it switches between 0 and auto are debatable, but
this seems good enough to me.  We should spec something, anyway.

(This is a subset of bug 15844, but it's much simpler than the general case,
where lengths are the big problem.  I think this should be resolved even if bug
15844 is pushed off.)

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Received on Wednesday, 7 March 2012 20:13:24 UTC