I read this tutorial on using the Grid Brush to turn an image into a pattern of rectangles, ellipses, etc. I tried this with the following steps:

Open Krita and open image as layer.

Create a new transparent paint layer on top of the image layer.

Click on the image layer to make the layer active.

Choose the Grid Brush from brush presets.

Uncheck, then check the box for "Sample Image layer" in Color Options. Expected: Grid Brush samples color from the image layer.

Turn off the visibility of the image layer.

Click the transparent paint layer and paint.

However, the Grid Brush seems to sample the current active layer rather than the layer which is active when "Sample Image layer" in Color Options is turned on. In my case, that means the Grid Brush samples the transparent pixels on the layer I want my pattern on. This doesn't really matter when I wish to turn an image into contiguous rectangles, since all I need to do is duplicate the image (for "nondestructive painting") and paint directly on the copy for a low-res version of the original image. However, this doesn't work for the other particle types (ellipse, line, etc.) because the original image shows through the gaps between particles.
I know that the tutorial I used is outdated, but the Grid Brush engine should still work the same way. Should I post this on the [url=bugs.kde.org]bug tracker[/url]?

This screenshot is too large for the site to display, so here's the link.
https://i.imgur.com/bbAmyvj.png

it didn't work that way even in Krita 3.0, which is the oldest appimage I can get. I think it would be more a wish request than a bug, since I can't get a version that it work the way it's described in the tutorial (although 3.0 was three years ago, so it's not that old...).

If you want just squares, better use Nearest Neighbour filter when first scaling down to a size you want, then scaling up to the original size (but make sure the scaling rate is integer).

I think Sample Input Layer doesn't have much use the way it is. Grid Brush (and especially Clone Brush) would be a lot more useful if they could sample layers other than the current active layer. That would, however, require major changes to the brush system and freehand brush tool options, and I agree that it's mainly a wish request on my part.

I did find a simple workaround to my issue that works with all Grid Brush particle types. I painted over my image with "Jitter borders" off using the Grid Brush with the particle type set to ellipses. To hide the original image showing through the gaps between the circles, I added a transparency mask to the image, filled the mask with black, and used the Grid Brush to paint white cutouts. That made the gaps transparent, so I flood filled a layer below.
Rather than using a transparency mask, I also could have made a grid of circles on a new transparent layer below my image and made my image inherit alpha. However, using a new layer would have increased the file size and memory usage.

The Clone Brush can sample from other layers, as noted in the Brush Editor window for Clone Engine brushes in the Painting Mode section.

Make the required source layer active then do a Ctrl+Click anywhere in that layer. Then make the target layer active and use Ctrl+Alt+Click on the visible source layer content. The Clone Brush will then clone content from source layer to target layer. (This works even if the source layer is obscured by any intermediate layers, which may be interesting/useful in some situations.)