Running minntty 3.7.7 via:

mintty.exe -B void -w max /usr/bin/bash --login -i

• fullscreening the window via right click, Full Screen.
• Running tmux with a split (either sshing to a remote host or running locally) with a vertical split escape (cnt-b) , :split -v
• if you minimize the mintty window via alt-space, Minimize (n), then restoring the window either by alt-tab or clicking the mintty icon in the Start Bar
• See the background window contents and maybe desktop data in the tmux split row.

Before minimize:

Minimized, then restored:

In the above the background window contents is Firefox. I've also tested this with Chrome, MS Team, and Outlook 365 mail, and explorer windows.

I update mintty often and I don't believe I remember seeing this before 3.7.7 but I'm not sure it was introduced with this release. I'm able to reproduce this with local cywgin tmux next-3.4, tmux 3.5a on FreeBSD and rockylinux 8 and 9 when sshing, and multiple Windows 11 environments.