Yes, I would say that this is a drawing error. Webkit seems to allow the box around the letter to crop the shadow when you start using the negative distance between letters.
It was easier for me to see the problem by increasing the size and making display: inline
to get a minimized frame around the letter.
http://jsfiddle.net/tMeuB/3/
Firefox
Starting from version 7.0.1
Pay attention to how the shadow remains when the distance from the minus letters increases from left to right.

Safari
Starting with version 5.1.1 (7534.51.22)
The same tests. See how the shadow is cropped as the distance between the letters increases. In frame 3, you can see how the algorithm computes it. I would suggest that clipping width = glyph width is the width of the glyph block.

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