try do a fresh clean install of windows, or just ignore the stretch as it only appears briefly when you start windows.  does your Windows desktop screen stay in normal aspect ratio?

I was thinking too that clean install will fix the problem. Will resetting PC instead of clean install works? On the other side I want to fix the problem without clean install. And my screen is in normal aspect ratio. Answer all questions in detail.

another user reported same issue.  try this 

runCommand Prompt as Administrator and copy/paste this:

bcdedit.exe -set {globalsettings} highestmode on

highestmode on Enables boot applications to use the highest graphical mode exposed by the firmware.

 Caution

Administrative privileges are required to use BCDEdit to modify BCD. Changing some boot entry options using the BCDEdit /set command could render your computer inoperable. 

Though Win10 has native graphics driver(s), you might try re-downloading the graphics driver.  https://www.dell.com/support/home/en-us/product-support/product/optiplex-790/drivers 

Please see screengrabs.  Skip the AMD Radeon 6350 driver, as that's for a specific mounted GPU.  Also, you'll see I chose "Windows 7 64-bit."  That's because there are no Win10 drivers from Dell for the 790.

None of these methods are working.

Re: bcdedit.exe -set {globalsettings} highestmode on

? does not work

To suppress all Windows UI elements (logo, status indicator, and status message) during startup, type the following cmd as admin:

bcdedit.exe -set {globalsettings} bootuxdisabled on

Sorry to hear that.  If I understand correctly, everything else is still working normally?  Is the window image your desktop image?  May be replace it with a photo?

@redxps630 

Not sure which reply you're replying to.  Easiest way I would think is change proportion of desktop.

Also, if a photo is set to desktop, window image doesn't appear.

What about? - Right click on desktop.  Choose "Personalize."  See screengrab of next window to see where to adjust how desktop fills in.

Please pardon all the questions, but is your Win10 on 21H1?

"need to re-enable secure boot in the bios" is a good reminder.

@Ahmed Malick  Hopefully something works, as this has also been somewhat of a learning experience.