Operating system: Windows-10-10.0.17134 64 Bits
Graphics card: GeForce GTX 1060/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 419.17
Broken: version: 2.80 (sub 72), branch: master (modified), commit date: 2019-05-25 01:24, hash: rB8e812504b4c5
Probably not a new issue.
If you right-click on something (take for example the Window->"Toggle Window Fullscreen" menu command) and choose "Assign Shortcut" then you get a little popup with a button (with an "A" on it??) and you need to pass the mouse over that button(??) in order for it to turn blue and change to "Press a Key" and then you can press a key to assign to the function or you can click off the pop-up to cancel it.
But if you make the dire mistake of clicking ON the blue "Press a Key" button then it assigns Left Click to the function(!). This results in the left click function having been reassigned at the Window level in the keymap (affecting almost everything), and if you try to right-click the command again, it's now disabled and shows "Non-keyboard Shortcut" and the tooltip says you have to use "User Preferences" (old terminology) to change it. It can be really hard to track these down in the keymap among all the other "Left Click" actions until you learn what it's doing.
Is it really reasonable to allow assigning non-keyboard shortcuts with the "Assign Shortcut" right-click menu command, and unmodified left-click in particular? and if we allow you to assign it, can't we allow you to un-assign or change it in the same way?