Comment 15 for bug 168943

To clarify:

As the bounding box used to determine the filter effects region must not include strokes the filter effects region for a purely horizontal or vertical line will be empty if filterUnits is objectBoundingBox, this would suggest that NOTHING should be shown. Inkscape shows the unblurred line (interestingly the filter does get handed a pixblock with a zero area...).

On the user experience side of things, having blur use filterUnits=objectBoundingBox leads to unexpected behaviour. (But using userSpaceOnUse could lead to other unexpected behaviour, disappearing objects, when styles get copy/pasted.)