I quickly figured out that clicking in a textbox and changing values with 'QWER' shortcut keys is what works.
This bug is old (a year, maybe more) and often accompanied by another problem - you can only see the active point on the curve, the curve itself is not drawn.
I know if I fiddle around with dragging the buttons the software can even crash.
I've tried all sorts of dev/nightly/stable/unstable versions over the past year. The OS matters, maybe even graphics card and driver. Windows XP has issues but is still popular, I mainly use Windows 2007 and see it there too.
The software developers should be able to fix this easily, I guess nobody brought it to their attention?
I could give more complete system specifications on request, but as the bug is so apparent it should be noticeable on most computers.