fix(windows/onivim#3157): Dead key fix (onivim#3166)

__Issue:__ On Windows, pressing a dead key (like <kbd>'</kbd>) twice would not emit any output. Curiously, composing works correctly (<kbd>'</kbd> + <kbd>a</kbd> -> `á`), and a dead character + space worked correctly too.

__Defect:__ In our keyboard-layout helper, we were calling `ToUnicodeEx`, which, confusingly, is actually side-effectful - it can change the state of the keyboard, in particular dead keys: https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-tounicodeex#parameters

__Fix:__ There is a new flag introduced in Win10 >1607 that causes that particular API call to  not change the keyboard state. With that fix, the dead keys behave as expected.

Fixes onivim#3157

CHANGES_CURRENT.md

@@ -12,6 +12,7 @@
 - #3146 - Vim: Fix command-line staying open when clicking the editor or file explorer (fixes #3031)
 - #3161 - Configuration: Turn soft word-wrap on by default (fixes #3161)
 - #3162 - Windows: Support opening UNC paths (fixes #3151)
+- #3166 - Windows: Fix dead key input (fixes #3157)
 
 ### Performance
 

manual_test/cases.md

@@ -192,6 +192,24 @@ __Pass:__
 - [ ] OSX
 - [ ] Linux
 
+## 7.4 Dead Keys
+
+Regresion test for #3157
+
+Prerequisite:
+- Install ENG-INTL keyboard layout
+
+- Switch keyboard layout to English (International)
+- Run Onivim 2
+- Enter dead key (') followed by space -> should type key
+- Press dead key twice (") - platform dependent, should output one or two instances of the key
+- Enter dead key (') followed by composing character (like a) - should get à
+
+__Pass:__
+- [ ] Win
+- [ ] OSX
+- [ ] Linux
+
 # 8. Workspace
 
 ## 8.1 Open Workspace via Command Palette

src/oni2-keyboard-layout/stubs/keyboard-layout-windows.c

@@ -7,6 +7,9 @@
 
 #define SPACE_SCAN_CODE 0x0039
 
+// Parameter to pass to ToUnicodeEx: https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-tounicodeex#parameters
+#define DONT_CHANGE_KEYBOARD_STATE 4
+
 #include <windows.h>
 #include <stdint.h>
 
@@ -87,7 +90,10 @@ CAMLprim value oni2_KeyboardLayoutInit() {
 int keyboardCharacterToUTF8(UINT keycode, UINT scancode, BYTE *keyboardState, char* dest, int destSize, HKL keyboardLayout) {
   wchar_t destUtf16[UTF16_BUFFER_SIZE];
 
-  int utf16Count = ToUnicodeEx(keycode, scancode, keyboardState, destUtf16, UTF16_BUFFER_SIZE, 0, keyboardLayout);
+  // CAREFUL: ToUnicodeEx has the side-effect of resetting dead key state, as well as others - see:
+  // https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-tounicodeex#remarks
+  // Fix #3157, for Windows 10 >1607 by passing bit 2 to `wFlags`
+  int utf16Count = ToUnicodeEx(keycode, scancode, keyboardState, destUtf16, UTF16_BUFFER_SIZE, DONT_CHANGE_KEYBOARD_STATE, keyboardLayout);
 
   if (utf16Count == -1) {
     return -1;