getting the height of all cells in treeview

[Stefan Kost <ensonic hora-obscura de>]

hi hi,

I subclass the treeview widget and override its expose handler to overlay custom
gfx. Therefore I need to know the height covered by the cells. I currently check
allocation and visible rect, but both are way too large if the tree has just a
few cells.
If the tree has a lot of rows, so that the scrolling takes effect the values are
correct.
Is there a way to get the heigth of all rows?

Thanks,
  Stefan
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