GIMP Version: 2.10.18
Operating System: Windows
OS Version: Windows 10
GIMP Experience: Intermediate Level

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I am confusing because "Normal" mode of layer blending in Gimp 2.10 is strange for me.
It is different from "Normal" mode of Gimp 2.8 which is based on the algorithm of Porter-Duff's over operation.

How is the "Normal" mode calculated in Gimp 2.10?

In the following, "LegacyOver" denotes the operation of "Normal" mode in Gimp 2.8
and "NewOver" denotes the operation in Gimp 2.10.

1) Gimp 2.8
fg RGBA(0,0,255,255) (Opacity 25%) "LegacyOver" bg RGBA(0,0,0,255)(Opacity 50%) = actual result1 RGBA(0,0,101,159)

2) Gimp 2.10
fg RGBA(0,0,255,255) (Opacity 25%) "NewOver" bg RGBA(0,0,0,255)(Opacity 50%) = actual result2 RGBA(0,0,170,159)

Additionally, about "Additon" mode of layer blending, I feel very strange for the behavior of Gimp 2.10 if background opacity is 0%. I also feel strange for the behavior of Gimp 2.8 when background opacity is not 0% and i posted this question as another topic.

Gimp 2.8 (Not the Legacy Mode of Gimp 2.10)
3) fg RGBA(0,0,255,255) (Opacity 100%) "LegacyAdd" bg RGBA(0,0,0,255)(Opacity 0%) = actual result3 RGBA(0,0,0,0)
4) fg RGBA(0,0,255,255) (Opacity 100%) "LegacyAdd" bg RGBA(0,0,0,255)(Opacity 1%) = actual result4 RGBA(0,0,127,2)

Gimp 2.10 (Legacy mode of Gimp 2.10 has the same behavior in the following)
5) fg RGBA(0,0,255,255) (Opacity 100%) "NewAdd" bg RGBA(0,0,0,255)(Opacity 0%) = actual result5 RGBA(0,0,255,255)
6) fg RGBA(0,0,255,255) (Opacity 100%) "NewAdd" bg RGBA(0,0,0,255)(Opacity 1%) = actual result6 RGBA(0,0,255,3)

Note that opacity(%) is different a little between Gimp 2.10 and 2.8.
For example, the result of 50% opacity of value 255 is 128 in Gimp 2.10 while the result is 127 in Gimp 2.8. It is the little things but I prefer 127 to 128 for 50% opacity of value 255.

I understand it.
I changed "Composite Space" into "RGB(perseptual)" in Gimp 2.10
and got the same result of Gimp 2.8.