diff --git a/Source/Modules/PrefMgr_Core.swift b/Source/Modules/PrefMgr_Core.swift
index 00fd83a9..5f258811 100644
--- a/Source/Modules/PrefMgr_Core.swift
+++ b/Source/Modules/PrefMgr_Core.swift
@@ -103,7 +103,11 @@ public class PrefMgr: PrefMgrProtocol {
   public var upperCaseLetterKeyBehavior: Int
 
   @AppProperty(key: UserDef.kTogglingAlphanumericalModeWithLShift.rawValue, defaultValue: true)
-  public var togglingAlphanumericalModeWithLShift: Bool
+  public var togglingAlphanumericalModeWithLShift: Bool {
+    didSet {
+      SessionCtl.theShiftKeyDetector.alsoToggleWithLShift = togglingAlphanumericalModeWithLShift
+    }
+  }
 
   @AppProperty(key: UserDef.kDisableShiftTogglingAlphanumericalMode.rawValue, defaultValue: false)
   public var disableShiftTogglingAlphanumericalMode: Bool
diff --git a/Source/Modules/SessionCtl_Core.swift b/Source/Modules/SessionCtl_Core.swift
index 82c56c17..48af84e2 100644
--- a/Source/Modules/SessionCtl_Core.swift
+++ b/Source/Modules/SessionCtl_Core.swift
@@ -173,6 +173,7 @@ extension SessionCtl {
     keyHandler.clear()  // 這句不要砍，因為後面 handle State.Empty() 不一定執行。
     keyHandler.ensureKeyboardParser()
 
+    Self.theShiftKeyDetector.alsoToggleWithLShift = PrefMgr.shared.togglingAlphanumericalModeWithLShift
     if isASCIIMode, !isCapsLocked, PrefMgr.shared.disableShiftTogglingAlphanumericalMode { isASCIIMode = false }
     if isCapsLocked { isASCIIMode = isCapsLocked }  // 同步 Caps Lock 狀態。