Storage empty threshold can't be dragged, click sets it to 0

Issue description

In the storage window, the "Empty with trucks when above" threshold (the red truck handle on the storage bar) can no longer be adjusted. Dragging the handle does nothing, and a single click immediately sets the threshold to 0, so the storage starts emptying everything. Typing a value is not possible either.
This used to work in an earlier version: I could drag the handle or enter a number.
No mods enabled in this save.

Reproduction steps

1. Open any Bulk Storage (tested with stone, threshold set to 200).
2. Click the red truck handle ("Empty with trucks when above") on the storage bar and try to drag it.
3. The handle does not follow the mouse. After the click the threshold is 0 (the whole bar turns red, tooltip shows 0).

Expected: the handle can be dragged to a new value, or a number can be typed.

Game version and build number

v0.8.7d b619

Operating system

Windows

Note: first screenshot = after the click (threshold 0), second = before (threshold 200).

Thanks for the report, fixed in the next patch.