Disable Save button in Preferences when there is nothing to change. This is when either:
The changes got explicitly reverted
User sets everything back as it was before opening Preferences, or does a factory reset (and settings were defaults previously)
Currently, the only way to make the button disabled is to completely close the window, then reopen it. This button creates that feeling of possibly-unwanted changes you done and forgot about, which will apply when you then change something useful.
The Save/Apply button gets enabled as soon as you change anything. If you change a setting and then manually change it again to what it used to be, the button will remain enabled.
There are Preferences pages where it would be too difficult to accurately work out if the config has changed and then changed again to be exactly the same as before, so it’s unlikely this will ever work differently.
(Please don’t bump threads unless they’re really old. We have hundreds to go through and it’ll take some time, but everything will be looked at.)