canvas proportions disrupted when image on canvas in box with button widget

[mantzaris]

There is an unexpected effect seen when trying to dock a canvas onto a Box, and then include a Button widget as well, or even when trying to duplicate the image into 2 segments:

img = load("hg1.png")
img2 = load("hg1.png")
c = canvas(UserUnit)
c2 = canvas(UserUnit)
bx = Box(:v)

win = Window("ok")
push!(win,bx)
push!(bx,c)
push!(bx,c2)

with the remaining code from the zoom/pan example remaining the same

zr = Signal(ZoomRegion(img))
# Interactivity: hold down Ctrl and then click-drag to select a
# region via rubberband. It updates `zr`.
zoomsigs = init_zoom_rubberband(c, zr)
# See also: init_pan_drag, init_zoom_scroll, init_pan_scroll
# You can turn on all of these for the same canvas
# Create a Signal containing a `view` of the image over the
# region of interest. This view will update anytime `zr` updates.
imgsig = map(zr) do r
    cv = r.currentview
    view(img, UnitRange{Int}(cv.y), UnitRange{Int}(cv.x))
end
## Turn on drawing for the canvas
# `draw`, when passed Signal(s), will cause the canvas to be updated
# whenever any of the input Signals updates. It will also redraw
# whenever the Canvas is resized.
redraw = draw(c, imgsig) do cnvs, image
    copy!(cnvs, image)
    # canvas adopts the indices of the zoom region. That way if we
    # zoom in further, we select the correct region.
    set_coordinates(cnvs, value(zr))
end

## Don't forget this!
Gtk.showall(win)

This effect where the displays for both Box components are disrupted is also visible when

bt = Button("ok")
push!(bx,bt)

how can the images on the canvas become stacked onto the layout object without a disruption?