If I set a box face for a button, and then set a line-space for the buffer, I expect the extra space between lines to be added outside of the box, like so:

+-------------------------+
|          Box            |
|                         |
+-------------------------+
       extra line space
+-------------------------+
|          Box            |
|                         |
+-------------------------+

However, right now, the bottom line of the box includes the extra line spacing, like so:

+-------------------------+
|          Box            |
|                         |
+                         +
|      extra line space   |
+-------------------------+
+-------------------------+
|          Box            |
|                         |
+-------------------------+

Screenshot: