It’s on the left side and the top. Never on the right or the bottom. Sometimes the blocks are black, other times they’re bright blue, green or yellow
It may also be worth mentioning that I hadn’t seen this behavior with Ver 3.4. It only started cropping up around 3.5+1600.
I’ve reported something similar, though for me it’s always colours and it’s triggered by lens correction:
@kofa There’s been a number of similar issues reported, but they all seem to be for different reasons. FWIW, I just tried to recreate your issue with on my Windows machine and found no error. Go figure.
@Dave22152 I just imported your images into my current darktable installation (ubuntu 20.04, darktable 3.5.0+2438~g941122872, Nvidia Quadro M1200 and nvidia drivers 460.80). Neither with opencl enabled nor with opencl disabled I can reproduce the issue on my side. Both images are looking fine and do not show any black or colored rectangles …
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Same here (Ubuntu 20.04, Nvidia GeForce RTX 2060, Nvidia driver 460.80). Not able to reproduce with your images and xmps, neither with opencl enabled nor disabled.
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Well, probably an issue related to Windows / Compiler / Optimization settings / other (shared) libraries / … There are so many possibilities. We need to identify some common components when those issues are popping up.
I saw the exact same issue on Arch Linux, so it’s not Windows specific. I’m not sure about the version of the compiler or libraries since the system has been updated since last seeing the problem on Apr 24.
I had issues with black squares all over the picture some days ago. This was related to negative RGB values (Black squares / rectangles while editing an image · Issue #9022 · darktable-org/darktable · GitHub) and it’s fixed now. Probably the issues showing up here have a similar reason, e.g. NaN or negative values … especially because it shows up on the left side, i.e. beginning of calculation for a line …
This acts more like a Heisenbug
That was my hope in bringing this forward. I’d be interested in any suggestions on how to investigate further, otherwise it’s just something to live with.
If you’re not seeing this in exported images, I’d say the problem lies somewhere in the pipeline to display. The internal image has to be scaled, cropped, and converted to the display profile before it is splatted on the screen, so there are a few places where this sort of thing might occur.
The blocks are rather big, so it’s possibly something isolated to tiling, with respect to the left display boundary, a single-channel NaN thing, black clipping thing, or somesort…
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A Heisenbug would change behaviour depending on e.g. running under a debugger, no? Maybe there’s some overlap between the categories.
‘A bug that disappears or alters its behavior when one attempts to probe or isolate it.’
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In my cases, there’s no tiling; I have 64 GB of RAM and set a rather high tiling limit – and images from my small LX7 (10 MPixels) are affected.
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Makes perfect sense. Do you see any way to test or isolate any of those possibilities? With the caveat that my programming skills are limited and out of date.
I think the dt dev who knows the display code will need to weigh in… I’m just another blind person feeling the elephant… 
I may have found a reproducible situation on Windows 10 where the Crop and Rotate module is the culprit. It has other issues as well. I’ll make a screencast in a moment.
Ping @hannoschwalm
I have no clue how to describe the effects exactly and what I would write in a darktable bug report, but there is more wrong with the crop and rotate module than just reproducibly creating border artifacts. Please take a look here:
For clarification: 3.5.0+2439~g9aefc3ceb-dirty on Windows 10, self-compiled in MSYS2. OpenCL is active. AMD Ryzen 3700X, 32 GB RAM and nVidia GeForce RTX 2070 Super.
The artifacts look very similar to what I’ve experienced, although mine pop up without the crop and rotate module activated.
In my case, the happen while zooming in or out. I wonder if the processes are related from a display pipeline perspective. I’ll try it with the C&R module later.
Later…
@Thanatomanic and @hannoschwalm, I had a chance to look at the screen cast more carefully and it the border artifacts look exactly like mine, however I tend to see them while zooming and can’t replicate them using the crop module (I’m not seeing any of the other issues from the screen cast either).
My question once again is if there is a connection between zooming the display and cropping that provides some additional clue that we’ve been searching for?