Sorry, I thought you were referring to an rslider. It looks like there is something amiss here. If you open up the two vslider SVGs from the Misc eamples folder you'll notive that the vslider is offset to start slightly higher than the dimensions of the page, giving the rect shape a minus Y value.
Sorry about this. It seems that I need to fix the issue. You may have to use a little trial and error for now. Note that if you have Cabbage open alongside your SVG editor, the changes you make in your graphics program will automatically take affect in Cabbage as soon as you move your mouse over the widget. At least this should make it relatively quick to find the right dimensions. But yeah, it still sucks. I'll add it to the growing list of things I need to work on!