Operating system: Windows-10-10.0.17763 64 Bits
Graphics card: GeForce GTX 1080 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 431.36
Broken: version: 2.81 (sub 2), branch: master, commit date: 2019-08-14 22:18, hash: rBcb7ead2e3b62
Short description of error
In Texture Paint mode - using External editor always grab screen in 8bit PNG even if texture on model 16bit int or even if texture is 32bit float!
Exact steps for others to reproduce the error
Create 1024x1024 float checkerboard texture.
Apply this texture to texture slot for shader and add this material to Suzanne.
Switch Object to Texture Pain mode.
In Blender preference define path to External app (in my case i using Photoshop, but can be any)
Define Screen Grab size in Texture Paint window - Tool -External
Press Quick Edit.
External app will open 8bit PNG.
This is sort of bug. Because not allow edit 16bit or 32bit textures using Blender.
Screen grab need to be at least 16bit or better if it will be depend on texture image properties: 8bit texture - 8 bit. 16 - 16, 16 float/32float - 16/32 float.
PNG also a sort of issue. Save and Load always extremely slow.
I think user should have a choice for external app file format.
PNG, Tiff, PSD, EXR...
Any that support alpha and 8/16bit