sd-web-ui-scripts

Mosaicing

The algorithm in action:

UI:

Download mosaic.py and place it in the stable-diffusion-webui/scripts folder, then restart webui.

You can find the algorithm in the img2img tab under "Scripts".

Patches will be resized for processing from patch size to whatever your width and height settings are and then scaled back down for the mosaic.
elliptical mask shape will lead to less visible seems/patches, but requires a larger overlap in order to not produce gaps
more overlap means more patches to process and thus longer computation
Use preview mode to view how a single patch would look after processing, before committing the entire image.
Use mask preview mode to check the coverage of the mask pattern for your image (white means a pixel will be regenerated, black parts of the image will remain untouched). You can combine this with the preview mode in order to examine a single mask.
"Upscale amount" will upscale the image using bilinear upscaling before processing. For better results, I'd recommend manual upscaling with a better upscaler.