Make the button on each delegate visible not only on hover to be more consistent with current status everywhere else.
Change the icon when the property is already added and this will just apply the suggestion. Using
edit-redoas a lack of a better
Keep properties sheet open when clicking on the button to add a new property. This allows to add several properties in a row. Clicking on the full row will keep the previous behavior for the simple case, closing the sheet so the new property can be edited right away
Previously it worked the same for both items, so the button was merely informative and the sheet would remain open only after detecting some window properties, which was a very hidden and confusing pattern.
Not CCBUGing on purpose, since while the idea is good, it comes from a rant-formed catch-it-all bug report.