Can't resize columns in middle pane

Resizing between the left/middle/right panes works fine, as does resizing the horizontal frames of the left pane, but mousing over the column borders in the middle pane does not yield a resizing icon, and grabbing column borders does not allow them to be dragged.

ff 3.0.14/Mac 10.5.8/Z 2.0b7.1

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