The Decrypt button of the SSH agent never activates.

[AlexpFr]

The reason is that the activation condition is: if (!key.comment().isEmpty() || !key.encrypted()).
But the comment is never empty because when opening the key we have:

if (key.comment().isEmpty() && !key.encrypted()) {
    key.setComment(username);
}

if (key.comment().isEmpty() && !key.encrypted()) {
    key.setComment(fileName);
}

I made a fix, I am preparing a PR.