Created attachment 134537 [details] Icons under the window border and Latte panel SUMMARY When you have a transparent window theme and Latte, the icons are kind of magnified by the window transparency. This creates a visible border between both. I think this is hard to solve, unless Latte's transparency settings are not controlled by Plasma's settings.
I can not understand from your screenshot the issue. Please a better screenshot that describes much clearer the issue.
Created attachment 134541 [details] Icons without the window
Created attachment 134542 [details] Transparency doesn't match with window border transparency
Created attachment 134543 [details] Transparency bug (bigger)
Ah sorry a litte much now maybe. I wouldn't really say it is a bug. The blur of Latte doesn't match well with the one from the window border. 1. The icons are kind of magnified by the window blur effect and the same does not happen under the Latte panel, because the icons are not under the Latte panel. 2. The background is not blurred the same. It is shifted a little to the right as shown on the last two pictures. It is really a minor problem. Most people probably would not care. Just my perfectionism I guess ;)
Created attachment 134544 [details] Only problems with icons with this background
any blur issue is relate to plasma kwin implementation... The blur and constast effects parameters are loaded from current plasma theme. I suppose that if you want a window decoration to match the plasma theme blur then you need to adjust the blur and background contrast values at the plasma theme file. It is found at "colors" file of each plasma theme.