I don't really like the idea of having three buttons. There are really only two outcomes: proceed, do not proceed. I think this message should be re worded anyway, something like:
"Remove the selected add-on from NVDA?" Then use OK | CANCEL.

I agree with @feerrenrut – Yes/No/Cancel is one button to much. I suggest the following:

• [Text] Do you really want to remove this add-on?
  WARNING: After restarting NVDA, this step cannot be reversed!
• [Buttons] "Yes, remove this add-on" and "Cancel" (has focus, allocated to the ESC key)

See also: Issue #10016

Question: What shall we do if such a removed add-on in the userConfig folder is still available in the systemConfig? Removing both at the same time would require admin privileges. Therefore I wouldn't prefer this.

@feerrenrut:

I don't really like the idea of having three buttons [...] this message should be re worded anyway

My first aim was to limit translation impact, but I agree OK/Cancel fits better here.

@XLTechie, @DrSooom:

Instead of yes/no/cancel, why not just do Remove and Cancel, with
Cancel as the default?

This sadly does not fit the standard Windows / WX message boxes.
You can have either Yes / No with an optional Cancel or OK with an optional Cancel.
In any case, Cancel is the only answer one can map to the escape key, No cannot.

@josephl:

Are you sure you wish to remove Screen Curtain add-on? This cannot be undone.

This would further reduce ambiguity and enhance user confidence.
I'll have a look at the user guide as well.

You can have either Yes / No with an optional Cancel or OK with an optional Cancel.

For what it's worth MessageBox is a helper function that wraps the MessageDialog class. If you use the class directly you can change the labels for the buttons with SetOKLabel etc.

Further testing of the OK/Cancel with Cancel as default felt pretty awkward.
We are so used of validating OK/Cancel with the enter or space keys and this Cancel as default forbids it, forcing to tab.
My proposal is then to stick with Yes/No and simply have No as default, as originally requested in #10015.
Nevertheless, the confirmation prompt now mentions the name of the add-on as suggested in #10087 (review)

Why do we actually want no as the default button? If a user tries to remove an add-on, in most cases he is expected to choose yes. If I press shift+delete in Windows Explorer, Yes is also chosen as the default button when removing a file/folder.

Why do we actually want no as the default button?

I must confess I also am not that much convinced, but feel like it does not arm either.
Main motivation here was to satisfy a request from a prolific reporter while we probably won't invest much time on somewhat related issues like #10025.

Please note the addition of the add-on name in the confirmation message, also included in this PR as suggested by @josephsl, does indeed improve user experience IMHO.

@LeonarddeR - For the average or less-experienced Windows user, in my opinion they'd more likely use the delete key by itself - in which case, if they accidentally delete a file/folder they have the option in most cases of recovering from the Recycle Bin. However, it's not currently possible to recover a deleted add-on so making No the default choice at least makes it less likely that an add-on will accidentally be deleted.

• Build (for testing PR)

See test results for failed build of commit 51f1a9f2bb

Damn, caught again by my own fix on checkPot...