What is Customise Menu supposed to do?

I've had the program for a long time. The 'Customize Menu' option on the context menu has always puzzled me but I've got along without it. But I want to learn to use it.

I see in the manual that it should show a 'normal' looking menu. But mine has always given just a blank window with a couple of checkboxes and a button. This is not what is described in the manual.

So I click 'Help'. I just get gobbledegook HTML/XML?

I've just created a new menu, but I want to control its width?

How do I do it, please?

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