diff --git a/Source/Modules/ControllerModules/InputSignal.swift b/Source/Modules/ControllerModules/InputSignal.swift
index 4bc1acbb..c972f704 100644
--- a/Source/Modules/ControllerModules/InputSignal.swift
+++ b/Source/Modules/ControllerModules/InputSignal.swift
@@ -110,8 +110,11 @@ enum KeyCodeBlackListed: UInt16 {
 }
 
 // CharCodes: https://theasciicode.com.ar/ascii-control-characters/horizontal-tab-ascii-code-9.html
-enum CharCode: UInt /* 16 */ {
-  case yajuusenpai = 114_514_191_191_810_893
+enum CharCode: UInt16 {
+  case yajuusenpaiA = 114
+  case yajuusenpaiB = 514
+  case yajuusenpaiC = 1919
+  case yajuusenpaiD = 810
   // CharCode is not reliable at all. KeyCode is the most appropriate choice due to its accuracy.
   // KeyCode doesn't give a phuque about the character sent through macOS keyboard layouts ...
   // ... but only focuses on which physical key is pressed.
diff --git a/Source/Modules/ControllerModules/InputState.swift b/Source/Modules/ControllerModules/InputState.swift
index 5931723e..989564fe 100644
--- a/Source/Modules/ControllerModules/InputState.swift
+++ b/Source/Modules/ControllerModules/InputState.swift
@@ -110,9 +110,9 @@ class InputState {
   /// Represents that the composing buffer is not empty.
   class NotEmpty: InputState {
     private(set) var composingBuffer: String
-    private(set) var cursorIndex: UInt
+    private(set) var cursorIndex: Int = 0 { didSet { cursorIndex = max(cursorIndex, 0) } }
 
-    init(composingBuffer: String, cursorIndex: UInt) {
+    init(composingBuffer: String, cursorIndex: Int) {
       self.composingBuffer = composingBuffer
       self.cursorIndex = cursorIndex
     }
@@ -129,7 +129,7 @@ class InputState {
     var poppedText: String = ""
     var tooltip: String = ""
 
-    override init(composingBuffer: String, cursorIndex: UInt) {
+    override init(composingBuffer: String, cursorIndex: Int) {
       super.init(composingBuffer: composingBuffer, cursorIndex: cursorIndex)
     }
 
@@ -156,7 +156,7 @@ class InputState {
 
   /// Represents that the user is marking a range in the composing buffer.
   class Marking: NotEmpty {
-    private(set) var markerIndex: UInt
+    private(set) var markerIndex: Int = 0 { didSet { markerIndex = max(markerIndex, 0) } }
     private(set) var markedRange: NSRange
     private var deleteTargetExists = false
     var tooltip: String {
@@ -176,7 +176,7 @@ class InputState {
         return ""
       }
 
-      let text = composingBuffer.substring(with: markedRange)
+      let text = composingBuffer.utf16SubString(with: markedRange)
       if markedRange.length < kMinMarkRangeLength {
         ctlInputMethod.tooltipController.setColor(state: .denialInsufficiency)
         return String(
@@ -221,11 +221,11 @@ class InputState {
     var tooltipForInputting: String = ""
     private(set) var readings: [String]
 
-    init(composingBuffer: String, cursorIndex: UInt, markerIndex: UInt, readings: [String]) {
+    init(composingBuffer: String, cursorIndex: Int, markerIndex: Int, readings: [String]) {
       self.markerIndex = markerIndex
       let begin = min(cursorIndex, markerIndex)
       let end = max(cursorIndex, markerIndex)
-      markedRange = NSRange(location: Int(begin), length: Int(end - begin))
+      markedRange = NSRange(location: begin, length: end - begin)
       self.readings = readings
       super.init(composingBuffer: composingBuffer, cursorIndex: cursorIndex)
     }
@@ -291,7 +291,7 @@ class InputState {
     }
 
     var chkIfUserPhraseExists: Bool {
-      let text = composingBuffer.substring(with: markedRange)
+      let text = composingBuffer.utf16SubString(with: markedRange)
       let (exactBegin, _) = composingBuffer.utf16CharIndex(from: markedRange.location)
       let (exactEnd, _) = composingBuffer.utf16CharIndex(
         from: markedRange.location + markedRange.length)
@@ -303,7 +303,7 @@ class InputState {
     }
 
     var userPhrase: String {
-      let text = composingBuffer.substring(with: markedRange)
+      let text = composingBuffer.utf16SubString(with: markedRange)
       let (exactBegin, _) = composingBuffer.utf16CharIndex(from: markedRange.location)
       let (exactEnd, _) = composingBuffer.utf16CharIndex(
         from: markedRange.location + markedRange.length)
@@ -314,7 +314,7 @@ class InputState {
 
     var userPhraseConverted: String {
       let text =
-        OpenCCBridge.crossConvert(composingBuffer.substring(with: markedRange)) ?? ""
+        OpenCCBridge.crossConvert(composingBuffer.utf16SubString(with: markedRange)) ?? ""
       let (exactBegin, _) = composingBuffer.utf16CharIndex(from: markedRange.location)
       let (exactEnd, _) = composingBuffer.utf16CharIndex(
         from: markedRange.location + markedRange.length)
@@ -332,7 +332,7 @@ class InputState {
     private(set) var candidates: [String]
     private(set) var useVerticalMode: Bool
 
-    init(composingBuffer: String, cursorIndex: UInt, candidates: [String], useVerticalMode: Bool) {
+    init(composingBuffer: String, cursorIndex: Int, candidates: [String], useVerticalMode: Bool) {
       self.candidates = candidates
       self.useVerticalMode = useVerticalMode
       super.init(composingBuffer: composingBuffer, cursorIndex: cursorIndex)
diff --git a/Source/Modules/ControllerModules/KeyHandler_HandleCandidate.swift b/Source/Modules/ControllerModules/KeyHandler_HandleCandidate.swift
index 7c631905..8b9e6eae 100644
--- a/Source/Modules/ControllerModules/KeyHandler_HandleCandidate.swift
+++ b/Source/Modules/ControllerModules/KeyHandler_HandleCandidate.swift
@@ -71,7 +71,7 @@ extension KeyHandler {
         }
         delegate!.keyHandler(
           self,
-          didSelectCandidateAt: Int(ctlCandidateCurrent.selectedCandidateIndex),
+          didSelectCandidateAt: ctlCandidateCurrent.selectedCandidateIndex,
           ctlCandidate: ctlCandidateCurrent
         )
         return true
@@ -260,11 +260,11 @@ extension KeyHandler {
         return false
       } else {  // 這裡不用「count > 0」，因為該整數變數只要「!isEmpty」那就必定滿足這個條件。
         if input.isEnd || input.emacsKey == vChewingEmacsKey.end {
-          if ctlCandidateCurrent.selectedCandidateIndex == UInt(candidates.count - 1) {
+          if ctlCandidateCurrent.selectedCandidateIndex == candidates.count - 1 {
             IME.prtDebugIntel("9B69AAAD")
             errorCallback()
           } else {
-            ctlCandidateCurrent.selectedCandidateIndex = UInt(candidates.count - 1)
+            ctlCandidateCurrent.selectedCandidateIndex = candidates.count - 1
           }
         }
       }
@@ -294,10 +294,10 @@ extension KeyHandler {
       }
 
       if index != NSNotFound {
-        let candidateIndex: UInt = ctlCandidateCurrent.candidateIndexAtKeyLabelIndex(UInt(index))
-        if candidateIndex != UInt.max {
+        let candidateIndex = ctlCandidateCurrent.candidateIndexAtKeyLabelIndex(index)
+        if candidateIndex != Int.max {
           delegate!.keyHandler(
-            self, didSelectCandidateAt: Int(candidateIndex), ctlCandidate: ctlCandidateCurrent
+            self, didSelectCandidateAt: candidateIndex, ctlCandidate: ctlCandidateCurrent
           )
           return true
         }
@@ -342,11 +342,11 @@ extension KeyHandler {
         }
 
         if shouldAutoSelectCandidate {
-          let candidateIndex: UInt = ctlCandidateCurrent.candidateIndexAtKeyLabelIndex(0)
-          if candidateIndex != UInt.max {
+          let candidateIndex = ctlCandidateCurrent.candidateIndexAtKeyLabelIndex(0)
+          if candidateIndex != Int.max {
             delegate!.keyHandler(
               self,
-              didSelectCandidateAt: Int(candidateIndex),
+              didSelectCandidateAt: candidateIndex,
               ctlCandidate: ctlCandidateCurrent
             )
             clear()
diff --git a/Source/Modules/ControllerModules/KeyHandler_States.swift b/Source/Modules/ControllerModules/KeyHandler_States.swift
index e4b963a8..ff5b2fb5 100644
--- a/Source/Modules/ControllerModules/KeyHandler_States.swift
+++ b/Source/Modules/ControllerModules/KeyHandler_States.swift
@@ -114,7 +114,7 @@ extension KeyHandler {
     let composedText = head + reading + tail
     let cursorIndex = composedStringCursorIndex + reading.utf16.count
 
-    let stateResult = InputState.Inputting(composingBuffer: composedText, cursorIndex: UInt(cursorIndex))
+    let stateResult = InputState.Inputting(composingBuffer: composedText, cursorIndex: cursorIndex)
 
     // Now we start weaving the contents of the tooltip.
     if tooltipParameterRef[0].isEmpty, tooltipParameterRef[1].isEmpty {
@@ -206,7 +206,7 @@ extension KeyHandler {
     if input.isCursorBackward || input.emacsKey == vChewingEmacsKey.backward, input.isShiftHold {
       var index = state.markerIndex
       if index > 0 {
-        index = UInt(state.composingBuffer.utf16PreviousPosition(for: Int(index)))
+        index = state.composingBuffer.utf16PreviousPosition(for: index)
         let marking = InputState.Marking(
           composingBuffer: state.composingBuffer,
           cursorIndex: state.cursorIndex,
@@ -227,7 +227,7 @@ extension KeyHandler {
     if input.isCursorForward || input.emacsKey == vChewingEmacsKey.forward, input.isShiftHold {
       var index = state.markerIndex
       if index < (state.composingBuffer.utf16.count) {
-        index = UInt(state.composingBuffer.utf16NextPosition(for: Int(index)))
+        index = state.composingBuffer.utf16NextPosition(for: index)
         let marking = InputState.Marking(
           composingBuffer: state.composingBuffer,
           cursorIndex: state.cursorIndex,
@@ -565,11 +565,11 @@ extension KeyHandler {
       // Shift + Right
       if currentState.cursorIndex < currentState.composingBuffer.utf16.count {
         let nextPosition = currentState.composingBuffer.utf16NextPosition(
-          for: Int(currentState.cursorIndex))
+          for: currentState.cursorIndex)
         let marking: InputState.Marking! = InputState.Marking(
           composingBuffer: currentState.composingBuffer,
           cursorIndex: currentState.cursorIndex,
-          markerIndex: UInt(nextPosition),
+          markerIndex: nextPosition,
           readings: currentReadings
         )
         marking.tooltipForInputting = currentState.tooltip
@@ -614,11 +614,11 @@ extension KeyHandler {
       // Shift + left
       if currentState.cursorIndex > 0 {
         let previousPosition = currentState.composingBuffer.utf16PreviousPosition(
-          for: Int(currentState.cursorIndex))
+          for: currentState.cursorIndex)
         let marking: InputState.Marking! = InputState.Marking(
           composingBuffer: currentState.composingBuffer,
           cursorIndex: currentState.cursorIndex,
-          markerIndex: UInt(previousPosition),
+          markerIndex: previousPosition,
           readings: currentReadings
         )
         marking.tooltipForInputting = currentState.tooltip
diff --git a/Source/Modules/ControllerModules/StringUtils.swift b/Source/Modules/ControllerModules/StringUtils.swift
index 4beae553..135f56f5 100644
--- a/Source/Modules/ControllerModules/StringUtils.swift
+++ b/Source/Modules/ControllerModules/StringUtils.swift
@@ -67,7 +67,7 @@ extension String {
     (self as NSString).expandingTildeInPath
   }
 
-  public func substring(with nsRange: NSRange) -> String {
-    (self as NSString).substring(with: nsRange)
+  public func utf16SubString(with range: NSRange) -> String {
+    (self as NSString).substring(with: range)
   }
 }
diff --git a/Source/Modules/IMEModules/ctlInputMethod.swift b/Source/Modules/IMEModules/ctlInputMethod.swift
index dd89e5b5..15360103 100644
--- a/Source/Modules/IMEModules/ctlInputMethod.swift
+++ b/Source/Modules/IMEModules/ctlInputMethod.swift
@@ -373,7 +373,7 @@ extension ctlInputMethod {
     // the selection range is where the cursor is, with the length being 0 and replacement range NSNotFound,
     // i.e. the client app needs to take care of where to put this composing buffer
     client.setMarkedText(
-      state.attributedString, selectionRange: NSRange(location: Int(state.cursorIndex), length: 0),
+      state.attributedString, selectionRange: NSRange(location: state.cursorIndex, length: 0),
       replacementRange: NSRange(location: NSNotFound, length: NSNotFound)
     )
     if !state.tooltip.isEmpty {
@@ -395,7 +395,7 @@ extension ctlInputMethod {
     // the selection range is where the cursor is, with the length being 0 and replacement range NSNotFound,
     // i.e. the client app needs to take care of where to put this composing buffer
     client.setMarkedText(
-      state.attributedString, selectionRange: NSRange(location: Int(state.cursorIndex), length: 0),
+      state.attributedString, selectionRange: NSRange(location: state.cursorIndex, length: 0),
       replacementRange: NSRange(location: NSNotFound, length: NSNotFound)
     )
 
@@ -420,7 +420,7 @@ extension ctlInputMethod {
     // the selection range is where the cursor is, with the length being 0 and replacement range NSNotFound,
     // i.e. the client app needs to take care of where to put this composing buffer
     client.setMarkedText(
-      state.attributedString, selectionRange: NSRange(location: Int(state.cursorIndex), length: 0),
+      state.attributedString, selectionRange: NSRange(location: state.cursorIndex, length: 0),
       replacementRange: NSRange(location: NSNotFound, length: NSNotFound)
     )
     show(candidateWindowWith: state, client: client)
@@ -437,7 +437,7 @@ extension ctlInputMethod {
     // the selection range is where the cursor is, with the length being 0 and replacement range NSNotFound,
     // i.e. the client app needs to take care of where to put this composing buffer
     client.setMarkedText(
-      state.attributedString, selectionRange: NSRange(location: Int(state.cursorIndex), length: 0),
+      state.attributedString, selectionRange: NSRange(location: state.cursorIndex, length: 0),
       replacementRange: NSRange(location: NSNotFound, length: NSNotFound)
     )
     show(candidateWindowWith: state, client: client)
@@ -553,7 +553,7 @@ extension ctlInputMethod {
     var cursor = 0
 
     if let state = state as? InputState.ChoosingCandidate {
-      cursor = Int(state.cursorIndex)
+      cursor = state.cursorIndex
       if cursor == state.composingBuffer.count, cursor != 0 {
         cursor -= 1
       }
@@ -581,9 +581,9 @@ extension ctlInputMethod {
     }
   }
 
-  private func show(tooltip: String, composingBuffer: String, cursorIndex: UInt, client: Any!) {
+  private func show(tooltip: String, composingBuffer: String, cursorIndex: Int, client: Any!) {
     var lineHeightRect = NSRect(x: 0.0, y: 0.0, width: 16.0, height: 16.0)
-    var cursor = Int(cursorIndex)
+    var cursor = cursorIndex
     if cursor == composingBuffer.count, cursor != 0 {
       cursor -= 1
     }
@@ -615,7 +615,7 @@ extension ctlInputMethod: KeyHandlerDelegate {
   ) {
     _ = keyHandler  // Stop clang-format from ruining the parameters of this function.
     if let controller = controller as? ctlCandidate {
-      ctlCandidate(controller, didSelectCandidateAtIndex: UInt(index))
+      ctlCandidate(controller, didSelectCandidateAtIndex: index)
     }
   }
 
@@ -651,34 +651,34 @@ extension ctlInputMethod: KeyHandlerDelegate {
 // MARK: -
 
 extension ctlInputMethod: ctlCandidateDelegate {
-  func candidateCountForController(_ controller: ctlCandidate) -> UInt {
+  func candidateCountForController(_ controller: ctlCandidate) -> Int {
     _ = controller  // Stop clang-format from ruining the parameters of this function.
     if let state = state as? InputState.ChoosingCandidate {
-      return UInt(state.candidates.count)
+      return state.candidates.count
     } else if let state = state as? InputState.AssociatedPhrases {
-      return UInt(state.candidates.count)
+      return state.candidates.count
     }
     return 0
   }
 
-  func ctlCandidate(_ controller: ctlCandidate, candidateAtIndex index: UInt)
+  func ctlCandidate(_ controller: ctlCandidate, candidateAtIndex index: Int)
     -> String
   {
     _ = controller  // Stop clang-format from ruining the parameters of this function.
     if let state = state as? InputState.ChoosingCandidate {
-      return state.candidates[Int(index)]
+      return state.candidates[index]
     } else if let state = state as? InputState.AssociatedPhrases {
-      return state.candidates[Int(index)]
+      return state.candidates[index]
     }
     return ""
   }
 
-  func ctlCandidate(_ controller: ctlCandidate, didSelectCandidateAtIndex index: UInt) {
+  func ctlCandidate(_ controller: ctlCandidate, didSelectCandidateAtIndex index: Int) {
     _ = controller  // Stop clang-format from ruining the parameters of this function.
     let client = currentClient
 
     if let state = state as? InputState.SymbolTable,
-      let node = state.node.children?[Int(index)]
+      let node = state.node.children?[index]
     {
       if let children = node.children, !children.isEmpty {
         handle(
@@ -693,7 +693,7 @@ extension ctlInputMethod: ctlCandidateDelegate {
     }
 
     if let state = state as? InputState.ChoosingCandidate {
-      let selectedValue = state.candidates[Int(index)]
+      let selectedValue = state.candidates[index]
       keyHandler.fixNode(value: selectedValue, respectCursorPushing: true)
 
       let inputting = keyHandler.buildInputtingState
@@ -718,7 +718,7 @@ extension ctlInputMethod: ctlCandidateDelegate {
     }
 
     if let state = state as? InputState.AssociatedPhrases {
-      let selectedValue = state.candidates[Int(index)]
+      let selectedValue = state.candidates[index]
       handle(state: .Committing(poppedText: selectedValue), client: currentClient)
       if mgrPrefs.associatedPhrasesEnabled,
         let associatePhrases = keyHandler.buildAssociatePhraseState(
diff --git a/Source/UI/CandidateUI/ctlCandidate.swift b/Source/UI/CandidateUI/ctlCandidate.swift
index f4333e9c..9fb076f5 100644
--- a/Source/UI/CandidateUI/ctlCandidate.swift
+++ b/Source/UI/CandidateUI/ctlCandidate.swift
@@ -38,11 +38,11 @@ public class CandidateKeyLabel: NSObject {
 }
 
 public protocol ctlCandidateDelegate: AnyObject {
-  func candidateCountForController(_ controller: ctlCandidate) -> UInt
-  func ctlCandidate(_ controller: ctlCandidate, candidateAtIndex index: UInt)
+  func candidateCountForController(_ controller: ctlCandidate) -> Int
+  func ctlCandidate(_ controller: ctlCandidate, candidateAtIndex index: Int)
     -> String
   func ctlCandidate(
-    _ controller: ctlCandidate, didSelectCandidateAtIndex index: UInt
+    _ controller: ctlCandidate, didSelectCandidateAtIndex index: Int
   )
 }
 
@@ -53,7 +53,7 @@ public class ctlCandidate: NSWindowController {
     }
   }
 
-  public var selectedCandidateIndex: UInt = .max
+  public var selectedCandidateIndex: Int = .max
   public var visible: Bool = false {
     didSet {
       NSObject.cancelPreviousPerformRequests(withTarget: self)
@@ -108,8 +108,8 @@ public class ctlCandidate: NSWindowController {
     false
   }
 
-  public func candidateIndexAtKeyLabelIndex(_: UInt) -> UInt {
-    UInt.max
+  public func candidateIndexAtKeyLabelIndex(_: Int) -> Int {
+    Int.max
   }
 
   /// Sets the location of the candidate window.
diff --git a/Source/UI/CandidateUI/ctlCandidateHorizontal.swift b/Source/UI/CandidateUI/ctlCandidateHorizontal.swift
index 17245e60..2e7ab7a7 100644
--- a/Source/UI/CandidateUI/ctlCandidateHorizontal.swift
+++ b/Source/UI/CandidateUI/ctlCandidateHorizontal.swift
@@ -27,7 +27,7 @@ CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 import Cocoa
 
 private class HorizontalCandidateView: NSView {
-  var highlightedIndex: UInt = 0
+  var highlightedIndex: Int = 0 { didSet { highlightedIndex = max(highlightedIndex, 0) } }
   var action: Selector?
   weak var target: AnyObject?
 
@@ -42,7 +42,9 @@ private class HorizontalCandidateView: NSView {
   private var candidateAttrDict: [NSAttributedString.Key: AnyObject] = [:]
   private var candidateWithLabelAttrDict: [NSAttributedString.Key: AnyObject] = [:]
   private var elementWidths: [CGFloat] = []
-  private var trackingHighlightedIndex: UInt = .max
+  private var trackingHighlightedIndex: Int = .max {
+    didSet { trackingHighlightedIndex = max(trackingHighlightedIndex, 0) }
+  }
 
   override var isFlipped: Bool {
     true
@@ -196,26 +198,27 @@ private class HorizontalCandidateView: NSView {
     }
   }
 
-  private func findHitIndex(event: NSEvent) -> UInt? {
+  private func findHitIndex(event: NSEvent) -> Int {
     let location = convert(event.locationInWindow, to: nil)
     if !bounds.contains(location) {
-      return nil
+      return NSNotFound
     }
     var accuWidth: CGFloat = 0.0
     for (index, elementWidth) in elementWidths.enumerated() {
       let currentWidth = elementWidth
 
       if location.x >= accuWidth, location.x <= accuWidth + currentWidth {
-        return UInt(index)
+        return index
       }
       accuWidth += currentWidth + 1.0
     }
-    return nil
+    return NSNotFound
   }
 
   override func mouseUp(with event: NSEvent) {
     trackingHighlightedIndex = highlightedIndex
-    guard let newIndex = findHitIndex(event: event) else {
+    let newIndex = findHitIndex(event: event)
+    guard newIndex != NSNotFound else {
       return
     }
     highlightedIndex = newIndex
@@ -223,7 +226,8 @@ private class HorizontalCandidateView: NSView {
   }
 
   override func mouseDown(with event: NSEvent) {
-    guard let newIndex = findHitIndex(event: event) else {
+    let newIndex = findHitIndex(event: event)
+    guard newIndex != NSNotFound else {
       return
     }
     var triggerAction = false
@@ -247,7 +251,7 @@ public class ctlCandidateHorizontal: ctlCandidate {
   private var candidateView: HorizontalCandidateView
   private var prevPageButton: NSButton
   private var nextPageButton: NSButton
-  private var currentPageIndex: UInt = 0
+  private var currentPageIndex: Int = 0
 
   public init() {
     var contentRect = NSRect(x: 128.0, y: 128.0, width: 0.0, height: 0.0)
@@ -364,24 +368,24 @@ public class ctlCandidateHorizontal: ctlCandidate {
     return true
   }
 
-  override public func candidateIndexAtKeyLabelIndex(_ index: UInt) -> UInt {
+  override public func candidateIndexAtKeyLabelIndex(_ index: Int) -> Int {
     guard let delegate = delegate else {
-      return UInt.max
+      return Int.max
     }
 
-    let result = currentPageIndex * UInt(keyLabels.count) + index
-    return result < delegate.candidateCountForController(self) ? result : UInt.max
+    let result = currentPageIndex * keyLabels.count + index
+    return result < delegate.candidateCountForController(self) ? result : Int.max
   }
 
-  override public var selectedCandidateIndex: UInt {
+  override public var selectedCandidateIndex: Int {
     get {
-      currentPageIndex * UInt(keyLabels.count) + candidateView.highlightedIndex
+      currentPageIndex * keyLabels.count + candidateView.highlightedIndex
     }
     set {
       guard let delegate = delegate else {
         return
       }
-      let keyLabelCount = UInt(keyLabels.count)
+      let keyLabelCount = keyLabels.count
       if newValue < delegate.candidateCountForController(self) {
         currentPageIndex = newValue / keyLabelCount
         candidateView.highlightedIndex = newValue % keyLabelCount
@@ -392,12 +396,12 @@ public class ctlCandidateHorizontal: ctlCandidate {
 }
 
 extension ctlCandidateHorizontal {
-  private var pageCount: UInt {
+  private var pageCount: Int {
     guard let delegate = delegate else {
       return 0
     }
     let totalCount = delegate.candidateCountForController(self)
-    let keyLabelCount = UInt(keyLabels.count)
+    let keyLabelCount = keyLabels.count
     return totalCount / keyLabelCount + ((totalCount % keyLabelCount) != 0 ? 1 : 0)
   }
 
@@ -409,7 +413,7 @@ extension ctlCandidateHorizontal {
     candidateView.set(keyLabelFont: keyLabelFont, candidateFont: candidateFont)
     var candidates = [String]()
     let count = delegate.candidateCountForController(self)
-    let keyLabelCount = UInt(keyLabels.count)
+    let keyLabelCount = keyLabels.count
 
     let begin = currentPageIndex * keyLabelCount
     for index in begin..<min(begin + keyLabelCount, count) {
diff --git a/Source/UI/CandidateUI/ctlCandidateVertical.swift b/Source/UI/CandidateUI/ctlCandidateVertical.swift
index 12eed454..de56c7c5 100644
--- a/Source/UI/CandidateUI/ctlCandidateVertical.swift
+++ b/Source/UI/CandidateUI/ctlCandidateVertical.swift
@@ -27,7 +27,7 @@ CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 import Cocoa
 
 private class VerticalCandidateView: NSView {
-  var highlightedIndex: UInt = 0
+  var highlightedIndex: Int = 0 { didSet { highlightedIndex = max(highlightedIndex, 0) } }
   var action: Selector?
   weak var target: AnyObject?
 
@@ -44,7 +44,9 @@ private class VerticalCandidateView: NSView {
   private var windowWidth: CGFloat = 0
   private var elementWidths: [CGFloat] = []
   private var elementHeights: [CGFloat] = []
-  private var trackingHighlightedIndex: UInt = .max
+  private var trackingHighlightedIndex: Int = .max {
+    didSet { trackingHighlightedIndex = max(trackingHighlightedIndex, 0) }
+  }
 
   override var isFlipped: Bool {
     true
@@ -201,26 +203,27 @@ private class VerticalCandidateView: NSView {
     }
   }
 
-  private func findHitIndex(event: NSEvent) -> UInt? {
+  private func findHitIndex(event: NSEvent) -> Int {
     let location = convert(event.locationInWindow, to: nil)
     if !bounds.contains(location) {
-      return nil
+      return NSNotFound
     }
     var accuHeight: CGFloat = 0.0
     for (index, elementHeight) in elementHeights.enumerated() {
       let currentHeight = elementHeight
 
       if location.y >= accuHeight, location.y <= accuHeight + currentHeight {
-        return UInt(index)
+        return index
       }
       accuHeight += currentHeight
     }
-    return nil
+    return NSNotFound
   }
 
   override func mouseUp(with event: NSEvent) {
     trackingHighlightedIndex = highlightedIndex
-    guard let newIndex = findHitIndex(event: event) else {
+    let newIndex = findHitIndex(event: event)
+    guard newIndex != NSNotFound else {
       return
     }
     highlightedIndex = newIndex
@@ -228,7 +231,8 @@ private class VerticalCandidateView: NSView {
   }
 
   override func mouseDown(with event: NSEvent) {
-    guard let newIndex = findHitIndex(event: event) else {
+    let newIndex = findHitIndex(event: event)
+    guard newIndex != NSNotFound else {
       return
     }
     var triggerAction = false
@@ -252,7 +256,7 @@ public class ctlCandidateVertical: ctlCandidate {
   private var candidateView: VerticalCandidateView
   private var prevPageButton: NSButton
   private var nextPageButton: NSButton
-  private var currentPageIndex: UInt = 0
+  private var currentPageIndex: Int = 0
 
   public init() {
     var contentRect = NSRect(x: 128.0, y: 128.0, width: 0.0, height: 0.0)
@@ -369,24 +373,24 @@ public class ctlCandidateVertical: ctlCandidate {
     return true
   }
 
-  override public func candidateIndexAtKeyLabelIndex(_ index: UInt) -> UInt {
+  override public func candidateIndexAtKeyLabelIndex(_ index: Int) -> Int {
     guard let delegate = delegate else {
-      return UInt.max
+      return Int.max
     }
 
-    let result = currentPageIndex * UInt(keyLabels.count) + index
-    return result < delegate.candidateCountForController(self) ? result : UInt.max
+    let result = currentPageIndex * keyLabels.count + index
+    return result < delegate.candidateCountForController(self) ? result : Int.max
   }
 
-  override public var selectedCandidateIndex: UInt {
+  override public var selectedCandidateIndex: Int {
     get {
-      currentPageIndex * UInt(keyLabels.count) + candidateView.highlightedIndex
+      currentPageIndex * keyLabels.count + candidateView.highlightedIndex
     }
     set {
       guard let delegate = delegate else {
         return
       }
-      let keyLabelCount = UInt(keyLabels.count)
+      let keyLabelCount = keyLabels.count
       if newValue < delegate.candidateCountForController(self) {
         currentPageIndex = newValue / keyLabelCount
         candidateView.highlightedIndex = newValue % keyLabelCount
@@ -397,12 +401,12 @@ public class ctlCandidateVertical: ctlCandidate {
 }
 
 extension ctlCandidateVertical {
-  private var pageCount: UInt {
+  private var pageCount: Int {
     guard let delegate = delegate else {
       return 0
     }
     let totalCount = delegate.candidateCountForController(self)
-    let keyLabelCount = UInt(keyLabels.count)
+    let keyLabelCount = keyLabels.count
     return totalCount / keyLabelCount + ((totalCount % keyLabelCount) != 0 ? 1 : 0)
   }
 
@@ -414,7 +418,7 @@ extension ctlCandidateVertical {
     candidateView.set(keyLabelFont: keyLabelFont, candidateFont: candidateFont)
     var candidates = [String]()
     let count = delegate.candidateCountForController(self)
-    let keyLabelCount = UInt(keyLabels.count)
+    let keyLabelCount = keyLabels.count
 
     let begin = currentPageIndex * keyLabelCount
     for index in begin..<min(begin + keyLabelCount, count) {