From Backgrounds & Borders: (http://www.w3.org/TR/2010/WD-css3-background-20100612/#the-box-shadow)
"If a spread distance is defined, the shadow is expanded outward or contracted inward by an operation equivalent to applying twice the absolute value of the spread value to a blur operation as defined below and thresholding the result such that for a positive spread distance all non-transparent pixels are given the full shadow color and for a negative spread distance all non-opaque pixels are made transparent. The UA may approximate this operation by taking an outward outset of the specified amount normal to the original shadow perimeter. Alternatively the UA may approximate the transformed shadow perimeter shape by outsetting (insetting, for inner shadows) the shadow's straight edges by the spread distance and increasing (decreasing, for inner shadows) and flooring at zero the corner radii by the same amount. (The UA may even combine these methods, using one method for outer shadows and another for inner ones.)"
In Gecko & Presto the corner radii is adjusted accordingly. In Webkit, it stays the same as the radii of the outer edge of the border. This only happens with outset shadows, as you can see in the 3rd div, inset shadows are rendered just fine.
(Originally discovered by David DeSandro, although I did a search and I don't think there's another bug report on this.)
Is this still an issue? It's hard to tell from the url.
Yes it is.
*** Bug 51386 has been marked as a duplicate of this bug. ***
Created attachment 77182 [details]
Testcase with negative spread
*** Bug 49913 has been marked as a duplicate of this bug. ***
Created attachment 79583 [details]
Hi mitz, as discussed at Bug 27417, I change the border-radius behavior in this patch.
Could you take a look when you have time?
Patch no longer applies.
*** Bug 57258 has been marked as a duplicate of this bug. ***
Created attachment 87630 [details]
Hi Simon, thank you for trying this!
I just updated the patch to ToT. Could you take a review for this when you have time?
Thanks in advance.
Comment on attachment 87630 [details]
View in context: https://bugs.webkit.org/attachment.cgi?id=87630&action=review
Could you fix the test so that the boxes don't overlap?
(In reply to comment #13)
> (From update of attachment 87630 [details])
> View in context: https://bugs.webkit.org/attachment.cgi?id=87630&action=review
> > LayoutTests/fast/box-shadow/spread-multiple-inset.html:1
> > +<html>
> Could you fix the test so that the boxes don't overlap?
Sure! I'll fix this and land the patch shortly.
Committed r82520: <http://trac.webkit.org/changeset/82520>
I think this may have regressed the 6th box in LayoutTests/fast/box-shadow/inset-box-shadows.html
I filed bug 58162.