BUG: 385097
FIXED-IN: 5.12.0
This patch to plasma-workspace is to improve the quality of images in notifications as reported in Bug 385097.
It is still not perfect because KIO::PreviewJob is used to create the thumbnail, and it only produces two sizes, 128px and 256px, so it is likely that a small image will be scaled up, blurring it somewhat.
I took two screenshots of the same Dolphin window. Here is the before, without the patch:
And the after, with the patch:
Better, but still not perfect.