Editor area produces gamma errors when scaling

[ff2000]

This issue dates back a while. I have an image that I spent quite some time editing until I was finally happy. Most of the editing took place in GIMP, Raws however were exported with RawTherapee.
The issue is that whenever I watched it in gwenview I thought it looks bad, way too dark. Went back to gimp, lightened it up, exported, little better, but not happy when viewing it next time.
Today I was finally fed up enough to dig deeper. And after opening the jpeg in geeqie, gwenview, gimp and RT I concluded that I have issues with gimp. At first. Luckily I found this:
https://phabricator.kde.org/D14491
Which linked to this
http://www.ericbrasseur.org/gamma.html?i=1
The image only is darker when I scale it down to view it. YES!

So after seeing this issue in RT with a JPEG file I tried one of the RAW files that were part of the image. Exported it to TIF, open in GIMP, scaled both down and here is the result:

Left is scaled in gimp, right is scaled in RT.
The right one is much darker.

This unfortunately means that I can't trust brightness/contrast, as long as I don't edit at 1:1.

RAW:
https://www.dropbox.com/s/h6e9j3nw30uwqcu/P1140252.RW2?dl=0
PP3:
https://www.dropbox.com/s/pe2sb64teutp3t1/P1140252.RW2.pp3?dl=0

EDIT:
I realized that I had local contrast activated, which will only show accurate at 1:1. Deactivated this and capture sharpening, exported, compared with gimp and this issue still persists.

[ff2000]

I took the "gamma_2_2.jpg" from http://www.ericbrasseur.org/gamma.html?i=1 and opened it in RT and darktable, then scaled the view to 50%, also exported 50% scaled down versions.
Gimp is the only software that got the scaling correctly.

I think this demonstrates the issue better than the image with the actual photo.

[heckflosse]

I confirm the issue. I will try resizing (in first step only for export) on linear data.

[Beep6581]

We dealt with resizing gamma issues before, see issues #981 #2270