Version:           svn trunk (using KDE 4.5.5) 
OS:                Linux

When blurring part of a paint layer that has transparency, the filter seems to hide anything that would be drawn on a previously transparent area and then overlay the original image that was to be blurred. In many cases this gives the appearence of nothing / very little happening at all.

Reproducible: Always

Steps to Reproduce:
1. Create a new image (say 1380 x 768 was what I used) with 100% transparency
2. Draw a line on the paint layer
3. Select filter -> Blur -> Gaussian blur -> 20 pixels

Actual Results:  
Very little appears to change at all.

Expected Results:  
The painted line blurs, with the blurred areas extending out by the same amount as the blur selected (eg 20pixels in the example above)

You'll notice that:
1. If you put an opaque layer underneath (say white) and merge the two, that blur works as expected on that layer (with no transparency).
2. Blur works as expected on shape layers that have transparency
3. Filter layers are affected in the same way
4. It doesn't seem to make any difference whether an area is selected, or just the whole layer is blurred
5. It seems to apply to more than one filer (eg both "Gaussian blur" and just "blur" do the same thing)