Some days ago, I displayed (how ?) a Gremlin (replacement?) window.
It does just have a default button, no Cancel.
And the default button is active only if you place some replacement text.
FWIW.
Xojo 2015r1
El Capitan
I checked the release notes since Xojo 2015r2 to Xojo 2017r1.1: no entry for Gremlin.
To check the release notes, I created (macOS) a folder and fill it with all release notes since/after my Xojo 2015r1 (licensed version) till the last version (2017r1.1.
Keep that folder open, frontmost.
Then, cmd-F, set the selection target to that folder and type something to search for.
Works fine, usually.