修复远程桌面软件按 modifier key 时产生幽灵 'a' 按键的问题 (#1116)

* Fix ghost 'a' keypress from remote desktop modifier key events

Some remote desktop software (e.g. Parsec, older Deskflow) sends
flagsChanged events via IOHIDPostEvent without setting
event.key.keyCode for modifier keys, causing it to default to 0
(kVK_ANSI_A). The osxKeycodeToRime function then maps keycode 0
to XK_a via additionalCodeMappings, producing a ghost 'a' keypress
whenever Shift, CapsLock, or other modifier keys are pressed.

This fix validates that event.keyCode corresponds to a known modifier
virtual keycode for flagsChanged events. When an invalid keycode is
detected (such as 0), the correct modifier keycode is inferred from
the changed modifier flags instead.

Fixes #825

Author: HunterTom94

sources/MacOSKeyCodes.swift

@@ -65,6 +65,30 @@ struct SquirrelKeycode {
     return UInt32(XK_VoidSymbol)
   }
 
+  static let modifierKeycodes: Set<UInt16> = [
+    UInt16(kVK_Shift), UInt16(kVK_RightShift),
+    UInt16(kVK_CapsLock),
+    UInt16(kVK_Control), UInt16(kVK_RightControl),
+    UInt16(kVK_Option), UInt16(kVK_RightOption),
+    UInt16(kVK_Command), UInt16(kVK_RightCommand),
+    UInt16(kVK_Function)
+  ]
+
+  static func inferModifierKeycode(from changes: NSEvent.ModifierFlags) -> UInt16? {
+    if changes.contains(.capsLock) {
+      return UInt16(kVK_CapsLock)
+    } else if changes.contains(.shift) {
+      return UInt16(kVK_Shift)
+    } else if changes.contains(.control) {
+      return UInt16(kVK_Control)
+    } else if changes.contains(.option) {
+      return UInt16(kVK_Option)
+    } else if changes.contains(.command) {
+      return UInt16(kVK_Command)
+    }
+    return nil
+  }
+
   private static let keycodeMappings: [Int: Int32] = [
     // modifiers
     kVK_CapsLock: XK_Caps_Lock,

sources/SquirrelInputController.swift

@@ -62,9 +62,22 @@ final class SquirrelInputController: IMKInputController {
       }
       // print("[DEBUG] FLAGSCHANGED client: \(sender ?? "nil"), modifiers: \(modifiers)")
       var rimeModifiers: UInt32 = SquirrelKeycode.osxModifiersToRime(modifiers: modifiers)
-      // For flags-changed event, keyCode is available since macOS 10.15
-      // (#715)
-      let rimeKeycode: UInt32 = SquirrelKeycode.osxKeycodeToRime(keycode: event.keyCode, keychar: nil, shift: false, caps: false)
+      // For flags-changed event, keyCode is available since macOS 10.15 (#715)
+      // Some remote desktop software (e.g. Parsec) sends flagsChanged events with
+      // keyCode defaulting to 0 (kVK_ANSI_A) instead of the actual modifier keycode,
+      // causing a ghost 'a' keypress. Validate and infer the correct keycode from
+      // the changed modifier flags when necessary. (#825)
+      var keyCode = event.keyCode
+      if !SquirrelKeycode.modifierKeycodes.contains(keyCode) {
+        guard let inferred = SquirrelKeycode.inferModifierKeycode(from: changes) else {
+          lastModifiers = modifiers
+          rimeUpdate()
+          handled = true
+          break
+        }
+        keyCode = inferred
+      }
+      let rimeKeycode: UInt32 = SquirrelKeycode.osxKeycodeToRime(keycode: keyCode, keychar: nil, shift: false, caps: false)
 
       if changes.contains(.capsLock) {
         // NOTE: rime assumes XK_Caps_Lock to be sent before modifier changes,