I have an issue which I Can reproduce on different Computers doing the following steps:
1 Create a comp 2d (a simple plane is enough)
2 Create a comp 3d (I put a Modell in it, probably a 2d Image will also make it)
3 Put something on your Editor timeline
4 Try: Switch between 3d comp and Editor timeline: The preview Shows everything like it was set up for (here: Full Resolution)
5 Now Switch to the comp and set the view to more Windows (here: 3)
6 Try again: Switching between 3d comp and Editor affects the sharpness of the Editor preview to something more blurry. Possible half Resolution or something.
7 Try also: Switch to the 2d comp and then to the timeline and the preview is sharp again as excepted.
I created a Video (quick and dirty, please do not bother about the bad positioning of the text Information): Link to video