Dragging "dockable" panels not working, causing loss of other functionality (Mac)

Observed on MacOS, 1.0alpha2 (e9298ea6, 2019-07-05)

Trying to drag a "dockable" panel like Fill and Stroke out of its "docked" position on the right hand side of the window does not succeed. It gives a "grayed out" rectangle as though it is trying to move, but it cannot be placed anywhere.

After trying this, much of the Inkscape UI is not usable. While menu items including Extensions still seem to work, it does not appear to be possible to select objects in the main window or select tools or use command-bar items.

Possibly related to inbox#631 (moved).