In the launcher, go to Game Settings, and ensure that your resolution is set correctly. Also switch between Borderless, fullscreen or windowed mode. Since we've also recently transitioned to DX11, you can try updating your video drivers.
If that doesn't help:, Please do a full clean re-install:
move any valued save games elsewhere, and user empire designs.txt from /Documents/Paradox Interactive/Stellaris/ if you have made any custom empires or races.
"uninstall" in Steam-Stellaris
manually delete both the Steam/SteamApps/common/Stellaris AND Documents/Paradox Interactive/Stellaris folders
re-install game in Steam, run a Steam Verify when done.
add stellaris.exe to the exceptions list of your antivirus app; ESPECIALLY if you have Windows Defender, add it to the Ransomware "Apps Allowed Through" list.
start the game with no mods active and test
In particular you need to make sure there are no files or folders left under either the Steam Stellaris or Documents Stellaris locations, before installing again.
If that still doesn't help:
DXDIAG is a program you run from the Windows search box on the task bar. After running it will open a window and start collecting info with a progress bar in the lower-left corner. When it completes click the 'save all information' button and save it to a file then attach that file here.
Please attach here your Documents/Paradox Interactive/Stellaris/settings.txt and pdx_settings.txt .
From your Documents/Paradox Interactive/Stellaris/logs/ folder, attach here system.log , error.log .
