[Emacs-diffs] emacs-25 401857e: Fix Alt-modified keys on some European MS-Windows keyboards

[Eli Zaretskii]

branch: emacs-25
commit 401857eda39c57bd59a1e3a1dee57fd6420eeab5
Author: Ilya Zakharevich <address@hidden>
Commit: Eli Zaretskii <address@hidden>

    Fix Alt-modified keys on some European MS-Windows keyboards
    
    * src/w32fns.c (deliver_wm_chars): If the reported character is
    ASCII, AND Meta modifier is a candidate, behave as if Meta is
    present, i.e. fall back to the legacy code.  (Bug#23251)
---
 src/w32fns.c |   38 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 37 insertions(+), 1 deletion(-)

diff --git a/src/w32fns.c b/src/w32fns.c
index b9002ba..c57b5a1 100644
--- a/src/w32fns.c
+++ b/src/w32fns.c
@@ -3154,9 +3154,45 @@ deliver_wm_chars (int do_translate, HWND hwnd, UINT msg, 
UINT wParam,
              SHORT r = VkKeyScanW (*b), bitmap = 0x1FF;
 
              FPRINTF_WM_CHARS((stderr, "VkKeyScanW %#06x %#04x\n", (int)r,
-                              wParam));
+                               wParam));
              if ((r & 0xFF) == wParam)
                bitmap = r>>8; /* *b is reachable via simple interface */
+             else
+               {
+                 /* VkKeyScanW() (essentially) returns the FIRST key with
+                    the specified character; so here the pressed key is the
+                    SECONDARY key producing the character.
+
+                    Essentially, we have no information about the "role" of
+                    modifiers on this key: which contribute into the
+                    produced character (so "are consumed"), and which are
+                    "extra" (must attache to bindable events).
+
+                    The default above would consume ALL modifiers, so the
+                    character is reported "as is".  However, on many layouts
+                    the ordering of the keys (in the layout table) is not
+                    thought out well, so the "secondary" keys are often those
+                    which the users would prefer to use with Alt-CHAR.
+                    (Moreover - with e.g. Czech-QWERTY - the ASCII
+                    punctuation is accessible from two equally [nu]preferable
+                    AltGr-keys.)
+
+                    SO:   Heuristic: if the reported char is ASCII, AND Meta
+                    modifier is a candidate, behave as if Meta is present
+                    (fallback to the legacy branch; bug#23251).
+
+                    (This would break layouts
+                    - delivering ASCII characters
+                    - on SECONDARY keys
+                    - with not Shift/AltGr-like modifier combinations.
+                    All 3 conditions together must be pretty exotic
+                    cases - and a workaround exists: use "primary" keys!) */
+                 if (*b < 0x80
+                     && (wmsg.dwModifiers
+                         & (alt_modifier | meta_modifier
+                            | super_modifier | hyper_modifier)))
+                   return 0;
+               }
              if (*type_CtrlAlt == 'a') /* Simple Alt seen */
                {
                  if ((bitmap & ~1) == 0) /* 1: KBDSHIFT */