Description of the new feature/enhancement

After a program has activated the alternate screen buffer, I want to view the primary screen buffer and its scrollback and copy some text from them to the clipboard.

For example, I run some other commands and then git commit, which starts the nano editor to edit the commit message. The editor activates the alternate screen buffer of the terminal. I then want to look at the output of the previous commands and copy parts of it to the commit message, but the last page of output is hidden behind the editor's screen.

Proposed technical implementation details (optional)

Add a command similar to Flip Screen (Alt+F12) in mintty of Git Bash, and let me bind that to a key combination. The command makes the primary screen buffer temporarily visible but keeps the alternate screen buffer active. It does not report this change to applications. I can then scroll back and forth, select text, and copy it to the clipboard. If I run the same command again or type something, then the alternate screen buffer becomes visible again. (If a program outputs something to the alternate screen buffer, I think the primary screen buffer should remain visible, but that is not very important.)

Implementing #3492 first might make this easier.