bounds outside level 0 width and height

[Ryan McLoughlin]

I've noticed that in some slides the "bounds" rectangle defined in the
openslide properties does not fall cleanly inside the outer rectangle
(defined by the slides' width and height properties).  For example I have a
MIRAX slide with:

width         = 87264
height        = 203931
bounds_x      = -271
bounds_y      = 86428
bounds_width  = 84001
bounds_height = 119454

Notice that bounds_x < 0 and also that bounds_y + bounds_height > height.

What does this mean?  Is there any extra data outside of the outer
rectangle that can be retrieved somehow?  I noticed that when I used vips'
autocrop feature to make a DZI file, like this:

vips dzsave input.mrxs[autocrop] output

the width and height of the DZI were 83730 and 117503, indicating that vips
simply ignored the parts of the bounds that were outside the outer
rectangle.  Is this the right approach to take?
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