StephanL wrote:
I've got a couple a picture where the horizon must be corrected.
After that step, if I want to keep a correct composition, there is a little white area in the corner.

is there a possibility to correct that (extend some background texture ?) within Lightroom or
is it mandatory to photoshop it ?

the "white" is whatever background color you have set at the time of the rotation. naturally, the area is a result of no image part outside of your image area. you may either crop it out or replace it with a matching part of the image from another area.

the greater the rotation, the more blank area will result. so a slight rotation and crop may be the easiest. it just depends on how much extra space you have in the frame. it is much more problematic for tight crops of the subject. seamless paper or single tone backgrounds are simple to extend or replace. much of it will depend on the rotation angle, the background and what area is available in your original image.