--- a/tests/test-extension.t	Tue Feb 05 11:17:11 2019 -0800
+++ b/tests/test-extension.t	Sat Feb 02 21:58:49 2019 -0800
@@ -823,7 +823,13 @@
       the working directory files are compared to its parent.
   
       The --per-file option runs the external program repeatedly on each file to
-      diff, instead of once on two directories.
+      diff, instead of once on two directories. By default, this happens one by
+      one, where the next file diff is open in the external program only once
+      the previous external program (for the previous file diff) has exited. If
+      the external program has a graphical interface, it can open all the file
+      diffs at once instead of one by one. See 'hg help -e extdiff' for
+      information about how to tell Mercurial that a given program has a
+      graphical interface.
   
       The --confirm option will prompt the user before each invocation of the
       external program. It is ignored if --per-file isn't specified.
@@ -905,6 +911,20 @@
     [diff-tools]
     kdiff3.diffargs=--L1 '$plabel1' --L2 '$clabel' $parent $child
   
+  If a program has a graphical interface, it might be interesting to tell
+  Mercurial about it. It will prevent the program from being mistakenly used in
+  a terminal-only environment (such as an SSH terminal session), and will make
+  'hg extdiff --per-file' open multiple file diffs at once instead of one by one
+  (if you still want to open file diffs one by one, you can use the --confirm
+  option).
+  
+  Declaring that a tool has a graphical interface can be done with the "gui"
+  flag next to where "diffargs" are specified:
+  
+    [diff-tools]
+    kdiff3.diffargs=--L1 '$plabel1' --L2 '$clabel' $parent $child
+    kdiff3.gui = true
+  
   You can use -I/-X and list of file or directory names like normal 'hg diff'
   command. The extdiff extension makes snapshots of only needed files, so
   running the external diff program will actually be pretty fast (at least