virtualbox.org

I'm using a 4K monitor, after installing 19.04, I change the dispaly to 200%
then the desktop could not display the background picture, only give me a white color.

anyone knows how to fix this problem?

Background picture of what? Are you running in seamless mode? I don't quite understand from your description what's going on here, can you try with a different, way more detailed approach?

In any case, we need to see a complete VBox.log, from a complete VM run, where the problem occurs:

• Start the VM from cold-boot (not from a paused or saved state) / Observe problem / Shutdown the VM (force close it if you have to).
• With the VM completely shut down (not paused or saved), right-click on the VM in the VirtualBox Manager and select "Show Log".
• Save only the first "VBox.log", ZIP it and attach it to your response. See the "Upload attachment" tab below the reply form.

I mean the background picture of the desktop, you can see the attachment file test.docx.
I've uploaded the log file, hope you can help me solve the problem, thanks.

here is the log file and the desktop picture

Please don't use a proprietary format (DOCX) to include two pictures that you could have easily put online. Please edit your message, delete the DOCX file and put the images inline instead. 640x480 should be a good size, not the 64x48 that you included in the DOCX, you can barely see anything.

So, if you run seamless, with a zoom of 200%, you don't get a transparent background. You should head to the bug tracker and either search for an appropriate bug, or open a new bug report where you'll attach your logs and other vital information.

WHEN I resize the desktop display to 100%, the desktop background picture can show correctly, as seen bellow:
But when I resize the display to 200%, the desktop background picture become a white background, the picture cannot show correctly, as seen bellow:

Thank you for including the pictures inline. Do you see how much better that looks?

As I said, it's most probably a bug. Please head to the bugtracker and file a bug report. "Exotic" features on Linux guests have actually a lower priority, because there aren't a lot of developers to deal with that, so I would stick with the 100% resolution and accept the fact. Simple...

See 3D support for X11 guests.