[editing] Comments on WebKit addRange implementation

The comments on https://dvcs.w3.org/hg/editing/raw-file/tip/editing.html#dom-selection-addrange say "Chrome 15 dev seems to ignore addRange() if there's already a range."

In case it's helpful, I wanted to note that this isn't quite the case. The WebKit implementation is here:
http://trac.webkit.org/browser/trunk/Source/WebCore/page/DOMSelection.cpp#L385

What that code does, if the selection has not already been cleared via e.g. `.removeAllRanges()`, is set the selection to the *intersection* of the old selection and range being "added".

Why? I have no idea. Union or extension I could see; intersection just seems bizarre. Hopefully this information is useful, though — even if it seems really hard to reconcile spec-wise with other implementations. If WebKit/Blink can't be fixed, maybe the behaviour of `addRange(range)` is simply "undefined" if `rangeCount` is 1…

regards,
-natevw