you can change rotation whenever you want - and this affects the whole image independently from cropping. I really can’t imagine why a rotation on a cropped area could differ from the rotation of the whole image.
If you have a valid use case for that, you simply can reorder pixel pipeline to do rotation after cropping…
As i wrote: better think in use cases instead of controls.
I’ll do a walk through as likely I wasn’t clear you will see what I mean I will do a short video….I know that I can change rotation whenever but not while cropped and displayed full screen as I said imagine you have two elements hypothetical but you level as I said based on the roof line…so nice and level for that view of the image…But now you do a substantial crop…maybe the roofline is not even in the image after you crop …Now maybe you have a sign post or mailbox or something in the crop that is crooked and a distraction…it would be nice to just draw a line on it in that cropped view at full screen display and just have it rotate and adjust the crop using the crop setting For this scenario at least as I worked it you have to go back to the rotation module……then the view jumps out of the full screen cropped display…you would need to zoom in to simulate the crop find the sign post and level it……so it is not an issue to complete the task but it seems like it could be smoother…I will have time later to make a short video and you can critique my steps in case I am missing something obvious….basically you can simulate it by leveling an image, do a substantial crop …now look at the image and decide you want to rotate it based on some element in the image….where do you go from there…. Back to rotate correct?? Then you lose your full screen cropped preview etc etc….
Long text for a quite simple scenario:
activate rotation module, zoom in on the small object you want to realign after cropping, set rotation, zoom out.
Of course this scenario can also be done with a crop frame allowing rotation as it’s done in lightroom. But the darktable way is far from being too complex…
You also have to make sure that the object you align on will actually orient the image as you want it:
while a rooftop may very well be horizontal in reality (if the building isn’t too old), that doesn’t mean its projection in the image should be horizontal. And there is no reason to believe a signpost is vertical in all cases.
i don’t see an issue there, you can rotate as you want, you aren’t limited to horizontal or vertical structures.
I don’t get your intention: there aren’t limits on how to rotate stuff in darktable or something that’s completely impractical to be done without the lightroom way.
You also don’t need to convince me, that the lightroom is a valid solution too, thats obvious …
Its not complex but its extra steps that should not be necessary. As a result of your cropping if you notice the need for a small adjustment to the rotation you shouldn’t have to go to another module and lose the current version of the displayed image likely fire up the pipeline a couple of extra times etc etc….I’m not saying it can be done…but something like this in ART….you crop….if you notice something need in alignment you select the level tool icon and make the adjustment and it gets transferred to the rotation module. No need fiddle around and zoom….
wait for next generation input, then define r+scroll (or whatever you want) to control rotation and you don’t need to switch between modules or drag some edges 
There’re more solutions possible than the lightroom one…
To be able to rotate and crop at the same time, you can also setup a shortcut for the “angle” setting for the Crop and Rotate module (darktable 3.6).
Go in the darktable settings => shortcuts => “processing modules” => crop and rotate => angle and set there a shortcut for the dynamic field.
Here I’ve set it to Shift+R. That means that, whenever I press Shift+R and I rotate my mouse wheel, the value of the angle will change and immediately be applied. This will allow you to crop and rotate with your mouse without having to move to the slider.
I always keep saying I will define some dynamic short cuts and configure a quick access panel to my liking but I never get around to it….crappy fall weather may help me find the time….
EDIT:
Thanks for the reminder…set it up in rotate and perspective with R scroll works great…Just had to create an auto preset as well for the rotate module as ti only has ± 10 degrees…I set it to ± 90 which should suffice for most of what I would ask of it…
The shortcut workaround is ok, but very slow. Might be related to how Darktable refreshes the image.
Yet it’s better than having to get back to the slider, thanks.
@MStraeten
I don’t mean to bother you, please don’t take it badly, but is there a problem suggesting a feature that is present in lightroom ? I might have gotten it wrong but you seem to be annoyed by this. Would it be different if that feature came from an other software ? (I think that’s quite a common way of scaling and rotating things)
fully agree with priort… Separating rotation and cropping would be a bad idea and makes more tedious the processing of images in real life (like wedding). I strongly encourage not to do it. Thanks
P_L is right I suggest, we all don’t care if LR or other software work that way : if Darktable community finds it useful, why not implementing it ? No reason to be different just for the sake of being different.
Its already done…new crop module only module and rotate and perspective redesigned and combined but the quick access panel and dynamic shortcuts can really work around most any issue…assigning scroll_r allows you to rotate from anywhere…just like using it for exposure with E to tweak that any time…
Lots of history of DT bashing and not being up to Lightroom standards or doing things the Lightroom way etc etc …will often yield this sentiment intended or otherwise in responses…
Yes I understand, I don’t mean to be that guy who comes and ask for a Lr clone.
For what it’s worth, I really prefer Darktable which allows much more control and understanding of the tools.
I found a bug though, I wanted to do a screen capture while cropping to illustrate the refreshing problem when using a shortcut, and it seems I cannot resize while recording my screen… Maybe it’s related to the screen capture program, I’ll see if that’s different with an other one
(Linux mint 20, simplescreenrecorder)
Mine is jerky if you mean that but I have a slow computer and DT is likely refreshing the pipeline as it rotates…it would be good to know if those with better hardware than me notice that if so there might be a way to optimize it…If this continues on crop and rotate we really should move some of this and continue to a new thread…
My hardware is fine, I guess. The images are big but not that much (6k*4k)
(Nvidia RTX 2060 and Ryzen 5 3600)
The bug after screen capture stayed (couldn’t crop, even after closing screen capture) until I discarded the whole history. I kept the XMP file, whenever I load it back I can’t crop the image. I’ll file a report on github with attachments.
No i don’t feel bothered; it’s ok if you file a feature request. But you should have a good use case since „intuitivity“ or „common use“ is usually not a good argument since thats mostly learned stuff. But it’s frequently used as an argument by lightroom user
The combined module will be deprecated with next release because separating them in the pixel pipe order gives some benefits, so a control combining rotation and crop will require quite good arguments.
If you have these to convince the developers - feel free…
of course put all the sliders you want in the quick access module and… access them quickly 
Isn’t “Not having to go back and forth from the module to the screen to tweak adjustments with the mouse” a suitable argument ? (or from a module to another in the next releases)
I’ll try and find a convincing example where this is most needed (according to my workflow I guess)
I do understand there are benefits of having both in separate modules though. Maybe if there’s a mouse control for rotation in the rotation module, shortcuts will be enough to tweak easily from one module to another.
