Drag a plan window to about 1400 pixels wide.
Click on the observations tab.
Drag the objects list to about 1/3 of the window width.
Try and drag the user fields list to be about 1/10 of the window width.
AstroPlanner won't let you.
Now drag the objects list to about 1/10 of the window width.
Note that you can now make the user fields list much narrower.

This is a tricky one to resolve. I believe it is a bug in the splitter widget used here.

Fixed. Not fixed as such, but improved. The complete fix would require some fairly major surgery, which is not going to happen at this stage.