diff -r a11237723332 -r 2282d8ac0fa9 mercurial/simplemerge.py
--- a/mercurial/simplemerge.py	Fri Jan 13 00:07:07 2023 +0100
+++ b/mercurial/simplemerge.py	Thu Jan 26 00:23:07 2023 +0100
@@ -512,6 +512,8 @@
     conflicts = False
     if mode == b'union':
         lines = _resolve(m3, (1, 2))
+    elif mode == b'union-other-first':
+        lines = _resolve(m3, (2, 1))
     elif mode == b'local':
         lines = _resolve(m3, (1,))
     elif mode == b'other':