From 6460cec4412a0f8bc5a16b91743423bef6c09e14 Mon Sep 17 00:00:00 2001
From: Alan Third <alan@idiocy.org>
Date: Tue, 18 Aug 2020 23:41:08 +0100
Subject: [PATCH] Handle fn + functions keys better on NS (bug#42915)

* doc/emacs/macos.texi (Mac / GNUstep Customization): Document the
caveat regarding fn and function keys.
* src/nsterm.m (EV_MODIFIERS2): Ignore ns_function_modifier if this is
a function key.
([EmacsView keyDown:]): Get rid of the code that unsets the
ns_function_modifier value in modifiers.
---
 doc/emacs/macos.texi |  4 ++++
 src/nsterm.m         | 23 ++++++++++-------------
 2 files changed, 14 insertions(+), 13 deletions(-)

diff --git a/doc/emacs/macos.texi b/doc/emacs/macos.texi
index 00daa8b35d..3fb625b417 100644
--- a/doc/emacs/macos.texi
+++ b/doc/emacs/macos.texi
@@ -143,6 +143,10 @@ Mac / GNUstep Customization
 may also be set to @code{left}, which means to use the same behavior as
 the corresponding left-hand key.
 
+Emacs is unable to detect when the @key{function} key is being used as
+a modifier with function keys. As a result Emacs will ignore the
+modifier set in @code{ns-function-modifier} with these keys.
+
 @subsection Frame Variables
 
 @table @code
diff --git a/src/nsterm.m b/src/nsterm.m
index 98c5b69d68..1c306c1b8e 100644
--- a/src/nsterm.m
+++ b/src/nsterm.m
@@ -402,7 +402,7 @@ - (NSColor *)colorUsingDefaultColorSpace
     hyper_modifier : 0)                                                 \
    | ((flags & NSEventModifierFlagShift) ?                              \
       shift_modifier : 0)                                               \
-   | ((flags & NS_FUNCTION_KEY_MASK)                                    \
+   | (((flags & NS_FUNCTION_KEY_MASK) && kind == QCordinary)            \
       ? parse_solitary_modifier (mod_of_kind (ns_function_modifier,     \
                                               kind))                    \
       : 0)                                                              \
@@ -6505,25 +6505,22 @@ untranslated characters (returned by the
          we have both shift-like and control-like modifiers because
          the NSEvent API doesn’t let us ignore only some modifiers.
          In that case we use UCKeyTranslate (ns_get_shifted_character)
-         to look up the correct character.  */
+         to look up the correct character.
 
-      /* EV_MODIFIERS2 uses parse_solitary_modifier on all known
+         EV_MODIFIERS2 uses parse_solitary_modifier on all known
          modifier keys, which returns 0 for shift-like modifiers.
          Therefore its return value is the set of control-like
-         modifiers.  */
-      Lisp_Object kind = fnKeysym ? QCfunction : QCordinary;
-      emacs_event->modifiers = EV_MODIFIERS2 (flags, kind);
+         modifiers.
 
-      /* Function keys (such as the F-keys, arrow keys, etc.) set
+         Function keys (such as the F-keys, arrow keys, etc.) set
          modifiers as though the fn key has been pressed when it
          hasn't.  Also some combinations of fn and a function key
          return a different key than was pressed (e.g. fn-<left> gives
-         <home>).  We need to unset the fn modifier in these cases.
-         FIXME: Can we avoid setting it in the first place?  */
-      if (fnKeysym && (flags & NS_FUNCTION_KEY_MASK))
-        emacs_event->modifiers
-          ^= parse_solitary_modifier (mod_of_kind (ns_function_modifier,
-                                                   QCfunction));
+         <home>).  EV_MODIFIERS2 avoids setting the the fn modifier in
+         these cases.  */
+
+      Lisp_Object kind = fnKeysym ? QCfunction : QCordinary;
+      emacs_event->modifiers = EV_MODIFIERS2 (flags, kind);
 
       if (NS_KEYLOG)
         fprintf (stderr, "keyDown: code =%x\tfnKey =%x\tflags = %x\tmods = %x\n",
-- 
2.26.1