The shader for calculating the blur for the GUI background uses box blur, even though it calculates linear blur. Linear blur looks a lot better than box blur, but is just not used. The shader should be updated to use this linear blur instead of the box blur that it currently uses.
The shader in question:
assets/minecraft/shaders/program/blur.fsh
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I wonder if it would be possible to include both as an in-game option, although linear is definitely preferable in my case.
This is most noticeable by the very prominent vertical and horizontal lines that stand out on your hotbar (and other UI elements with sharp edges, like text in the chat for example), especially if your GUI size is a bit larger:
[media]
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After consideration, the issue is being closed as Invalid.
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this is not a feature request, it is a bug report. i am reporting the fact that the blur is not being used properly, please actually read the bug report before closing it.
Can confirm