[Gimp-user] How to apply Filters -> Generic -> Erode to a layer?

[Thomas Nyberg <ml+gimp-user-list thomasnyberg com>]

Hello,

It's a bit easier to understand my question with an example. If you open 
a new image and add in some paint tool "squiggles" (i.e. just draw some 
random stuff with the mouse), then you can "thicken" these lines after 
they are already in place by using Filters -> Generic -> Erode. If you 
click that a few times the lines themselves will spread out a little. 
This does as I expect.

Now what I want is for this to work with a layer which I'll explain with 
another example. If you create a new image as before, but you also first 
select Layer -> New Layer and then use the paint tool to add some little 
squiggles. But in this case if you do Filters -> Generic -> Erode, 
nothing happens to the lines at all. If I select Layer -> Merge Down and 
then run Filters -> Generic -> Erode again, it thickens as expected.

My question is how do I thicken the layer before merging it down? Is 
this expected behavior or a bug? Thanks for any help. Yell at me if my 
question is confusing.

Cheers,
Thomas

P.S. I'm running GIMP 2.10.8 installed directly from the debian repos 
for buster (i.e. debian 10).