Comment 49 for bug 327863

Comment on attachment 373011
patch part 2 (works now): just clamp computed border/padding to containing box

Sorry I didn't get to this much sooner.

Reducing the border and/or padding in general is incorrect. The correct rendering of the testcase:

<div style="width: 3px">
  <div style="border: 5px;height:20px"></div>
</div>

is to have the inner div's border-box still be 10px wide (and 30px tall), overflowing its parent. (I hope you'll find that that's true across browsers, although I admit I haven't checked.)

It looks to me like this patch changes that, at least in some cases.