diff --git a/Packages/Qwertyyb_ShiftKeyUpChecker/Sources/ShiftKeyUpChecker/ShiftKeyUpChecker.swift b/Packages/Qwertyyb_ShiftKeyUpChecker/Sources/ShiftKeyUpChecker/ShiftKeyUpChecker.swift
index 6181ae84..e37f46b9 100644
--- a/Packages/Qwertyyb_ShiftKeyUpChecker/Sources/ShiftKeyUpChecker/ShiftKeyUpChecker.swift
+++ b/Packages/Qwertyyb_ShiftKeyUpChecker/Sources/ShiftKeyUpChecker/ShiftKeyUpChecker.swift
@@ -3,7 +3,6 @@
 // This code is released under the MIT license (SPDX-License-Identifier: MIT)
 
 import AppKit
-import Carbon
 
 private extension Date {
   static func - (lhs: Date, rhs: Date) -> TimeInterval {
@@ -12,6 +11,8 @@ private extension Date {
 }
 
 public struct ShiftKeyUpChecker {
+  // MARK: - 威注音輸入法專有部分
+
   public init(useLShift: Bool = false, useRShift: Bool = false) {
     toggleWithLShift = useLShift
     toggleWithRShift = useRShift
@@ -24,7 +25,7 @@ public struct ShiftKeyUpChecker {
 
   public var enabled: Bool { toggleWithLShift || toggleWithRShift }
 
-  private var checkModifier: NSEvent.ModifierFlags { NSEvent.ModifierFlags.shift }
+  private var checkModifier: NSEvent.ModifierFlags { .shift }
   private var checkKeyCode: [UInt16] {
     var result = [UInt16]()
     if toggleWithLShift { result.append(lShiftKeyCode) }
@@ -32,43 +33,31 @@ public struct ShiftKeyUpChecker {
     return result
   }
 
-  private let delayInterval = 0.3
+  // MARK: - 與業火五筆共用的內容
 
+  /// 實現邏輯基本上是相同的，只是威注音這邊的行文風格習慣可能與業火五筆有不同。
+
+  private let delayInterval = 0.3
+  private var previousKeyCode: UInt16?
   private var lastTime: Date = .init()
 
-  private var shiftIsBeingPressed = false
-
-  private mutating func checkModifierKeyUp(event: NSEvent) -> Bool {
-    guard checkKeyCode.contains(event.keyCode) else { return false }
-    if event.type == .flagsChanged,
-       event.modifierFlags.intersection(.deviceIndependentFlagsMask) == .init(rawValue: 0),
-       Date() - lastTime <= delayInterval, shiftIsBeingPressed
-    {
-      // modifier keyup event
-      lastTime = Date(timeInterval: -3600 * 4, since: Date())
-      return true
-    }
-    return false
+  private mutating func registerModifierKeyDown(event: NSEvent) {
+    var isKeyDown: Bool = event.type == .flagsChanged
+    // 注意：ModifierFlags 是 OptionSet，在使用 contains 時會在給定參數是「空集合」的時候返回 true（明明你可能想要 false）。
+    isKeyDown = isKeyDown && event.modifierFlags.intersection(.deviceIndependentFlagsMask) == checkModifier
+    isKeyDown = isKeyDown && checkKeyCode.contains(event.keyCode)
+    lastTime = isKeyDown ? .init() : .init(timeInterval: .infinity * -1, since: Date())
+    previousKeyCode = isKeyDown ? event.keyCode : nil
   }
 
-  private mutating func checkModifierKeyDown(event: NSEvent) -> Bool {
-    let isKeyDown =
-      event.type == .flagsChanged
-        && checkModifier.contains(event.modifierFlags.intersection(.deviceIndependentFlagsMask))
-        && checkKeyCode.contains(event.keyCode)
-    if isKeyDown {
-      // modifier keydown event
-      lastTime = Date()
-      if event.modifierFlags.intersection(.deviceIndependentFlagsMask) == .shift { shiftIsBeingPressed = true }
-    } else {
-      lastTime = Date(timeInterval: -3600 * 4, since: Date())
-      shiftIsBeingPressed = false
-    }
-    return false
-  }
-
-  // To confirm that the shift key is "pressed-and-released".
+  // To confirm that only the shift key is "pressed-and-released".
   public mutating func check(_ event: NSEvent) -> Bool {
-    checkModifierKeyUp(event: event) || checkModifierKeyDown(event: event)
+    var met: Bool = event.type == .flagsChanged
+    met = met && checkKeyCode.contains(event.keyCode)
+    met = met && event.keyCode == previousKeyCode // 檢查 KeyCode 一致性。
+    met = met && event.modifierFlags.intersection(.deviceIndependentFlagsMask).isEmpty
+    met = met && Date() - lastTime <= delayInterval
+    _ = met ? lastTime = Date(timeInterval: .infinity * -1, since: Date()) : registerModifierKeyDown(event: event)
+    return met
   }
 }