mac/input: add support for dead key reporting

dead keys like tilde and accents were not reported because they returned
an empty string from the NSEvent.

try to retrieve dead key with alternative Carbon API when an empty key
is reported.

osdep/mac/input_helper.swift

@@ -107,7 +107,9 @@ class InputHelper: NSObject {
             }
 
             guard let chars = event.characters, let charsNoMod = event.charactersIgnoringModifiers else { return false }
-            let key = (useAltGr() && event.modifierFlags.contains(.optionRight)) ? chars : charsNoMod
+            var key = (useAltGr() && event.modifierFlags.contains(.optionRight)) ? chars : charsNoMod
+            if key.isEmpty { key = mapDeadKey(event) }
+            if key.isEmpty { return false }
             key.withCString {
                 var bstr = bstr0($0)
                 putKey(bstr_decode_utf8(bstr, &bstr), modifiers: event.modifierFlags, type: event.type)
@@ -230,6 +232,23 @@ class InputHelper: NSObject {
         return Int32(buttonMapping[button] ?? SWIFT_MBTN9 + Int32(button - 5));
     }
 
+    func mapDeadKey(_ event: NSEvent) -> String {
+        let keyboard = TISCopyCurrentKeyboardInputSource().takeRetainedValue()
+        let layoutPtr = TISGetInputSourceProperty(keyboard, kTISPropertyUnicodeKeyLayoutData)
+        let layoutData = unsafeBitCast(layoutPtr, to: CFData.self)
+        let layout = unsafeBitCast(CFDataGetBytePtr(layoutData), to: UnsafePointer<UCKeyboardLayout>.self)
+        let maxLength = 2
+        let modifiers: UInt32 = UInt32(event.modifierFlags.rawValue >> 16) & 0xff
+        var deadKeyState: UInt32 = 0
+        var length = 0
+        var chars = [UniChar](repeating: 0, count: maxLength)
+        let err = UCKeyTranslate(layout, event.keyCode, UInt16(kUCKeyActionDisplay), modifiers,
+                                 UInt32(LMGetKbdType()), 0, &deadKeyState, maxLength, &length, &chars)
+
+        if err != noErr || length < 1 { return "" }
+        return String(utf16CodeUnits: chars, count: length)
+    }
+
     @objc func open(files: [String], append: Bool = false) {
         lock.withLock {
             guard let input = input else { return }