Inkscape can not blur Bezier line if it is connected to the outer end of a spiral

Question

I have added a Bezier line to the outer end of a spiral. (See comment for details.) After that I selected both, colorized them and gave them a small amount of blur (1.5%).

The problem is, that only the spiral gets blury, but not the Bezier line.

Nevertheless, if I move one node of the Bezier line somewhere else to the outer path of the spiral, select them both and apply a blur, both get blured. I moved the node with the Edit paths by nodes tool and enabled Enable snapping -> Snap notes or handles -> Snap to paths.

Next I can drag one node of the blured Bezier line to the cusp node of the blured spiral by hand and level the line horizontal, by entering the same y-value for the right node that the left node has. Now I can select them both and can change the blur of them simulataneously. By that I can achieve the result that I could not get by the first method. (See EDIT)

I would like to know, how I can blur both objects at the same time without all the effort described above, if this is not a bug.

user@hostname:~$ apt-cache show inkscape 
Package: inkscape
[…]
Architecture: amd64
Version: 0.48.3.1-1ubuntu6.1
[…]

[EDIT]: After messing arround with the problem for another while I realised, that the blur of the line only disapears, if I level the two nodes of the line exactly one the same y-values with copy and paste.
In my first try I entered the y-value of the left node of the line in the "y-value box" (at top) of the right line-node by hand, what probably results in not exact the same numbers as copy and paste does.
In my second try to recreate the problem, I copied and pasted the y-value from the left line-node to the right line-node and as soon as I did that the blur of the line disapears.

I could not find any information that this behaviour is intended by the developers. Thus I guess this is a bug and I will report it to them and write about the progress in here.

Answer 1

I got the following answer from the Inkscape developer ~suv:

Technically not a bug - a horizontal (or vertical) line has only one dimension, and the filter effects region thus too (i.e. the blur is not visible).

Workarounds:
- Combine the line with the spiral (the bounding box of the resulting paths as two dimensions), and blur the combined path.
- Group the line with the spiral, and blur the group, instead of the individual objects.

https://bugs.launchpad.net/inkscape/+bug/1187728