Lion overlay scrollers can't be clicked in Hemlock windows

[rme]

On Lion, it appears that overlay-style scroll bars can't be clicked in Hemlock editor windows. The clicks appear to go to the underlying text view, and they make a selection rather than scrolling the window content.

To reproduce:

• open a file large enough to need scrolling
• scroll by gesture so that the scroll bars appear
• try to drag the scrollbar

Observe that the scrollbar cannot be grabbed.

Using "legacy" scrollbars works fine.

1.8-dev-r15098, Mac OS X 10.7.2. I see the behavior in both 32-bit and 64-bit lisps.