characters on Desktop that can't be removed

The floating text is a rendering glitch caused by macOS's drag-and-drop mechanism getting interrupted. This occasionally occurs in macOS Tahoe when a brief lag stalls the system's compositing engine right as you release a text selection.

How did it get there?

Normally, highlighting text and dragging it to your desktop creates a snippet file called a "Text Clipping". When you accidentally or quickly drag text across the screen, macOS generates a temporary semi-transparent "ghost" overlay under your cursor to visually show what you are moving. 

If the application you are dragging from (like Mail) or the system window server drops the connection before the animation completes, the visual layer detaches and gets permanently stuck on your screen. Because it is just a stray layer of raw pixels floating on top of the window compositor, it doesn't exist as a real file in Finder or Terminal.

Since killing the Finder doesn't work (as Finder doesn't manage the active window rendering engine), you can clear it instantly using one of these workarounds without rebooting (are they 'elegant'? you decide...)

Kill the WindowServer

The WindowServer is the core process that handles everything you see on screen, including these stuck visual layers. Restarting it will immediately clear the ghost artifacts without a full machine reboot, though it will log you out of your desktop session.

• Open Terminal (via Spotlight or Applications > Utilities).
• Type the following command and hit return:

sudo killall -9 WindowServer

• Type your Mac password when prompted (nothing will appear on the screen as you type), then press return.
• Your screens will go black for a second and return you with a fresh display canvas.

Change your Screen Resolution (Forces a screen refresh)

Forcing macOS to recalculate the screen layout will reset all graphic layers and purge the phantom text box.

• Go to the Apple menu  > System Settings.
• Click Displays on the left sidebar.
• Select any resolution different from your current one (e.g., toggle from "Default" to "More Space").
• Once the screen flashes and changes, switch it right back to your original preferred resolution.

More on that from Apple: Change your Mac display’s resolution - Apple Support

Thank you again. Very interesting. But before your post, it happened a THIRD time (I'm not liking Tahoe, I guess), and your "sudo killall -9 WindowServer" worked a treat. It does log you out, as you noted, so that's a bit of a disadvantage, but not that big a deal. Next time I'll try the screen refresh.

Well first of all this is the very first time I’ve ever heard of something like this. That means it’s likely something particular to your installation, not a bug in macOS Tahoe. Try restarting your Mac in Safe Mode. Safe Mode often clears up glitches like this.

Start up your Mac in safe mode - Apple Support

If the glitch clears restart in normal mode and hopefully the issue is gone.

Like I said, the terminal command (killing window server) works well. But the Change

your Screen Resolution (Forces a screen refresh) does not have any effect.

It's not clear to me, either. Definitely a glitch, but the only 'floating text' that I have seen 'stuck' is a tooltip, like what was discussed in this thread.

Unwanted and unrelated "Tooltips" at mous… - Apple Community

That doesn't sound like what you described in your initial post ("characters in my e-mail text"), though a screenshot of the issue may have helped.

This is the process I am talking about that seems to be leaving something behind for you. If you select text then click and hold the selection briefly then start to drag, you make a text clipping.

Not sure what Safe Mode accomplishes. As I said, if I restart, the characters disappear. So restarting in Safe Mode doesn't do anything that a regular restart doesn't. Now, I remember occasionally seeing this in Sonoma, so it's not a Tahoe issue. It was just curious that it happened within a few minutes of completing my Tahoe update. The issue is whether there is some simpler way to clear these characters than by going through a restart.

Wow. That's a very complete and useful story. A stray layer of raw pixels. Thank you! Next time it happens, I'll try your fixes. As I say, I saw it rarely in Sonoma, but I've been using Tahoe for a few hours and I've already seen it happen twice.

Hmmm. Let's see. I tried doing a text clipping to floating text on the Desktop as you instructed, and it didn't work. I select text (I just selected the text "This is the process" from your post), clicked on it and dragged to the Desktop, I just end up with a "text clipping" file with the title "This is the process". That file is easily trashed. I don't get floating text. So it still isn't quite clear to me how floating text happens.