stylesheets.js:getRuleLocation can sometimes incorrectly compute the start line for a rule. In particular I saw it happen with an inline style sheet where the first rule was @supports. In this case the check here: https://dxr.mozilla.org/mozilla-central/source/toolkit/devtools/server/actors/stylesheets.js#963 ... fails, leading it to use the second rule to compute the absolute start line. I think the fix is to change the check to use Ci.nsIDOMCSSRule rather than nsIDOMCSSStyleRule. I have a fix for this; just need to write a test case.

Here's the patch without tests.

Now with test.

Comment on attachment 8645735 [details] [diff] [review] widen instanceof test in getRuleLocation Review of attachment 8645735 [details] [diff] [review]: ----------------------------------------------------------------- Seems fine to me.

The try run showed that this is flawed; some rules don't use a "{" for their location but rather the token following the @-keyword. Then it occurred to me that even expanding the heuristic to check this would be wrong, because an invalid @-keyword would be recognized by a naive scanner but be ignored by the real parser. Then I found bug 1013814. I've decided to try to fix this the real way.