In Windows, the leading byte for multibyte characters are set to upper

controls/edit.c

@@ -772,7 +772,8 @@ static LRESULT WINAPI EditWndProc_common( HWND hwnd, UINT msg,
 
                 strng[0] = wParam >> 8;
                 strng[1] = wParam & 0xff;
-                MultiByteToWideChar(CP_ACP, 0, strng, 2, &charW, 1);
+                if (strng[0]) MultiByteToWideChar(CP_ACP, 0, strng, 2, &charW, 1);
+                else MultiByteToWideChar(CP_ACP, 0, &strng[1], 1, &charW, 1);
 		EDIT_WM_Char(es, charW);
 		break;
             }

dlls/user/message.c

@@ -347,7 +347,8 @@ static WPARAM map_wparam_AtoW( UINT message, WPARAM wparam )
             WCHAR wch;
             ch[0] = (wparam >> 8);
             ch[1] = (wparam & 0xff);
-            MultiByteToWideChar(CP_ACP, 0, ch, 2, &wch, 1);
+            if (ch[0]) MultiByteToWideChar(CP_ACP, 0, ch, 2, &wch, 1);
+            else MultiByteToWideChar(CP_ACP, 0, &ch[1], 1, &wch, 1);
             wparam = MAKEWPARAM( wch, HIWORD(wparam) );
         }
         break;
@@ -384,9 +385,10 @@ static WPARAM map_wparam_WtoA( UINT message, WPARAM wparam )
             WCHAR wch = LOWORD(wparam);
             BYTE ch[2];
 
-            ch[1] = 0;
-            WideCharToMultiByte( CP_ACP, 0, &wch, 1, ch, 2, NULL, NULL );
-            wparam = MAKEWPARAM( (ch[0] << 8) | ch[1], HIWORD(wparam) );
+            if (WideCharToMultiByte( CP_ACP, 0, &wch, 1, ch, 2, NULL, NULL ) == 2)
+                wparam = MAKEWPARAM( (ch[0] << 8) | ch[1], HIWORD(wparam) );
+            else
+                wparam = MAKEWPARAM( ch[0], HIWORD(wparam) );
         }
         break;
     }

windows/defwnd.c

@@ -871,9 +871,9 @@ LRESULT WINAPI DefWindowProcA( HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam
 	    CHAR    chChar1 = (CHAR)( (wParam>>8) & 0xff );
 	    CHAR    chChar2 = (CHAR)( wParam & 0xff );
 
-	    SendMessageA( hwnd, WM_CHAR, (WPARAM)chChar1, lParam );
-	    if ( IsDBCSLeadByte( chChar1 ) )
-		SendMessageA( hwnd, WM_CHAR, (WPARAM)chChar2, lParam );
+	    if (chChar1)
+		SendMessageA( hwnd, WM_CHAR, (WPARAM)chChar1, lParam );
+	    SendMessageA( hwnd, WM_CHAR, (WPARAM)chChar2, lParam );
 	}
 	break;
     case WM_IME_KEYDOWN:

windows/winproc.c

@@ -797,7 +797,10 @@ INT WINPROC_MapMsg32ATo32W( HWND hwnd, UINT msg, WPARAM *pwparam, LPARAM *plpara
             WCHAR wch;
             ch[0] = (*pwparam >> 8);
             ch[1] = *pwparam & 0xff;
-            MultiByteToWideChar(CP_ACP, 0, ch, 2, &wch, 1);
+            if (ch[0])
+                MultiByteToWideChar(CP_ACP, 0, ch, 2, &wch, 1);
+            else
+                MultiByteToWideChar(CP_ACP, 0, &ch[1], 1, &wch, 1);
             *pwparam = MAKEWPARAM( wch, HIWORD(*pwparam) );
         }
         return 0;
@@ -1092,9 +1095,10 @@ INT WINPROC_MapMsg32WTo32A( HWND hwnd, UINT msg, WPARAM *pwparam, LPARAM *plpara
             WCHAR wch = LOWORD(*pwparam);
             BYTE ch[2];
 
-            ch[1] = 0;
-            WideCharToMultiByte( CP_ACP, 0, &wch, 1, ch, 2, NULL, NULL );
-            *pwparam = MAKEWPARAM( (ch[0] << 8) | ch[1], HIWORD(*pwparam) );
+            if (WideCharToMultiByte( CP_ACP, 0, &wch, 1, ch, 2, NULL, NULL ) == 2)
+                *pwparam = MAKEWPARAM( (ch[0] << 8) | ch[1], HIWORD(*pwparam) );
+            else
+                *pwparam = MAKEWPARAM( ch[0], HIWORD(*pwparam) );
         }
         return 0;