deselect hotkey

Ctl-A selects all. What deselects all?

One quickly learns that clicking the mouse again after selecting [as mentioned] undoes it. Also pressing the Esc key may work within programs such as MSPaint.

I'm thinking File Explorer (details view). Having selected all, shift-space deselects many -- but not all. I didn't find shift-space listed on any shortcut guide, so not sure whether that's a kink in my PC. Odd that there is no way to do that. Maybe an AutoHotkey macro?

The following is not the solution to your question; it is only a method that I use:

Assuming that I use Ctrl+A to select all files/folders within File Explorer, and assuming that I am only using the keyboard (not the mouse), I would press the Down Arrow. That just selects the file that is under whatever was selected singularly last.

And I just tried your Shift+Space method. The same thing happens here. It seems to select the last file that was singularly selected, or the first item listed assuming that you didn't select any item.

Example: I opened a folder that has 4 files in it. I pressed Ctrl+A and that selected all four of the items. I pressed Shift+Space and that selected the first file.
I clicked on the 3rd file, pressed Ctrl+A to select all, then pressed Shift+Space. The 3rd file is the only file that is now selected.

Thanks all. Selecting a single file after ^A does simplify the process.