seemingly, the Box with PACK_SHRINK allocates a zero height to the rulerCanvas initally,
which is correct at that point, since the widgets are not yet realised.
However, when we later set_size() on the rulerCanvas, the enclosing Box should reflow.
It does indeed if the child widget is a button or something similar, however,
somehow this reflowing does not work when we set_size on the canvas.
A workaround is to place a new set_size_request().
TODO: do this more precisely, and only on the rulerCanvas. To the contrary,
the mainCanvas is placed into a scolling-pane and thus does not need a size-Request.
Moreover, the latter automatically communicates with the hadjustment() / vadjustment() of
the enclosing scrollbars.
Ichthyostega
fc5eaf857c