diff --git a/McBopomofoTests/PreferencesTests.swift b/McBopomofoTests/PreferencesTests.swift
index 905f827c..6c4ebc56 100644
--- a/McBopomofoTests/PreferencesTests.swift
+++ b/McBopomofoTests/PreferencesTests.swift
@@ -156,3 +156,102 @@ class PreferencesTests: XCTestCase {
     }
 
 }
+
+class CandidateKeyValidationTests: XCTestCase {
+    func testEmpty() {
+        do {
+            try Preferences.validate(candidateKeys: "")
+            XCTFail("exception not thrown")
+        } catch(Preferences.CandidateKeyError.empty)  {
+        } catch {
+            XCTFail("exception not thrown")
+        }
+    }
+
+    func testSpaces() {
+        do {
+            try Preferences.validate(candidateKeys: "    ")
+            XCTFail("exception not thrown")
+        } catch(Preferences.CandidateKeyError.empty)  {
+        } catch {
+            XCTFail("exception not thrown")
+        }
+    }
+
+    func testInvalidKeys() {
+        do {
+            try Preferences.validate(candidateKeys: "中文字元")
+            XCTFail("exception not thrown")
+        } catch(Preferences.CandidateKeyError.invalidCharacters)  {
+        } catch {
+            XCTFail("exception not thrown")
+        }
+    }
+
+    func testInvalidLatinLetters() {
+        do {
+            try Preferences.validate(candidateKeys: "üåçøöacpo")
+            XCTFail("exception not thrown")
+        } catch(Preferences.CandidateKeyError.invalidCharacters)  {
+        } catch {
+            XCTFail("exception not thrown")
+        }
+    }
+
+    func testSpaceInBetween() {
+        do {
+            try Preferences.validate(candidateKeys: "1 2 3 4")
+            XCTFail("exception not thrown")
+        } catch(Preferences.CandidateKeyError.containSpace)  {
+        } catch {
+            XCTFail("exception not thrown")
+        }
+    }
+
+    func testDuplicatedKeys() {
+        do {
+            try Preferences.validate(candidateKeys: "aabbccdd")
+            XCTFail("exception not thrown")
+        } catch(Preferences.CandidateKeyError.duplicatedCharacters)  {
+        } catch {
+            XCTFail("exception not thrown")
+        }
+    }
+
+    func testTooShort1() {
+        do {
+            try Preferences.validate(candidateKeys: "abc")
+            XCTFail("exception not thrown")
+        } catch(Preferences.CandidateKeyError.tooShort)  {
+        } catch {
+            XCTFail("exception not thrown")
+        }
+    }
+
+    func testTooShort2() {
+        do {
+            try Preferences.validate(candidateKeys: "abcd")
+        } catch {
+            XCTFail("Should be safe")
+        }
+    }
+
+    func testTooLong1() {
+        do {
+            try Preferences.validate(candidateKeys: "qwertyuiopasdfgh")
+            XCTFail("exception not thrown")
+        } catch(Preferences.CandidateKeyError.tooLong)  {
+        } catch {
+            XCTFail("exception not thrown")
+        }
+    }
+
+    func testTooLong2() {
+        do {
+            try Preferences.validate(candidateKeys: "qwertyuiopasdfg")
+        }
+        catch {
+            XCTFail("Should be safe")
+        }
+    }
+}
diff --git a/Packages/CandidateUI/Sources/CandidateUI/CandidateController.swift b/Packages/CandidateUI/Sources/CandidateUI/CandidateController.swift
index c583b9bb..cc287ea6 100644
--- a/Packages/CandidateUI/Sources/CandidateUI/CandidateController.swift
+++ b/Packages/CandidateUI/Sources/CandidateUI/CandidateController.swift
@@ -130,7 +130,7 @@ public class CandidateController: NSWindowController {
 
         // bottom beneath the screen?
         if adjustedPoint.y - windowSize.height < screenFrame.minY {
-            adjustedPoint.y = windowTopLeftPoint.y + adjustedHeight + windowSize.height
+            adjustedPoint.y = screenFrame.minY + adjustedHeight + windowSize.height
         }
 
         // top over the screen?
diff --git a/Packages/CandidateUI/Tests/CandidateUITests/HorizontalCandidateControllerTests.swift b/Packages/CandidateUI/Tests/CandidateUITests/HorizontalCandidateControllerTests.swift
index 1aa3003d..585355aa 100644
--- a/Packages/CandidateUI/Tests/CandidateUITests/HorizontalCandidateControllerTests.swift
+++ b/Packages/CandidateUI/Tests/CandidateUITests/HorizontalCandidateControllerTests.swift
@@ -19,6 +19,35 @@ class HorizontalCandidateControllerTests: XCTestCase {
         }
     }
 
+    func testPositioning1() {
+        let controller = HorizontalCandidateController()
+        let mock = Mock()
+        controller.delegate = mock
+        controller.keyLabels = ["1", "2", "3", "4"]
+        controller.reloadData()
+        controller.visible = true
+        controller.set(windowTopLeftPoint: NSPoint(x: -100, y: -100), bottomOutOfScreenAdjustmentHeight: 10)
+        let exp = expectation(description: "wait")
+        _ = XCTWaiter.wait(for: [exp], timeout: 0.2)
+        XCTAssert (controller.window?.frame.minX ?? -1 >= 0)
+        XCTAssert (controller.window?.frame.minY ?? -1 >= 0)
+    }
+
+    func testPositioning2() {
+        let controller = HorizontalCandidateController()
+        let mock = Mock()
+        controller.delegate = mock
+        controller.keyLabels = ["1", "2", "3", "4"]
+        controller.reloadData()
+        controller.visible = true
+        let screenRect = NSScreen.main?.frame ?? NSRect.zero
+        controller.set(windowTopLeftPoint: NSPoint(x: screenRect.maxX + 100, y: screenRect.maxY + 100), bottomOutOfScreenAdjustmentHeight: 10)
+        let exp = expectation(description: "wait")
+        _ = XCTWaiter.wait(for: [exp], timeout: 0.2)
+        XCTAssert (controller.window?.frame.maxX ?? CGFloat.greatestFiniteMagnitude <= screenRect.maxX)
+        XCTAssert (controller.window?.frame.maxY ?? CGFloat.greatestFiniteMagnitude <= screenRect.maxY )
+    }
+
     func testReloadData() {
         let controller = HorizontalCandidateController()
         let mock = Mock()
diff --git a/Packages/CandidateUI/Tests/CandidateUITests/VerticalCandidateControllerTests.swift b/Packages/CandidateUI/Tests/CandidateUITests/VerticalCandidateControllerTests.swift
index afc54c41..0f11378c 100644
--- a/Packages/CandidateUI/Tests/CandidateUITests/VerticalCandidateControllerTests.swift
+++ b/Packages/CandidateUI/Tests/CandidateUITests/VerticalCandidateControllerTests.swift
@@ -19,6 +19,35 @@ class VerticalCandidateControllerTests: XCTestCase {
         }
     }
 
+    func testPositioning1() {
+        let controller = HorizontalCandidateController()
+        let mock = Mock()
+        controller.delegate = mock
+        controller.keyLabels = ["1", "2", "3", "4"]
+        controller.reloadData()
+        controller.visible = true
+        controller.set(windowTopLeftPoint: NSPoint(x: -100, y: -100), bottomOutOfScreenAdjustmentHeight: 10)
+        let exp = expectation(description: "wait")
+        _ = XCTWaiter.wait(for: [exp], timeout: 0.2)
+        XCTAssert (controller.window?.frame.minX ?? -1 >= 0)
+        XCTAssert (controller.window?.frame.minY ?? -1 >= 0)
+    }
+
+    func testPositioning2() {
+        let controller = HorizontalCandidateController()
+        let mock = Mock()
+        controller.delegate = mock
+        controller.keyLabels = ["1", "2", "3", "4"]
+        controller.reloadData()
+        controller.visible = true
+        let screenRect = NSScreen.main?.frame ?? NSRect.zero
+        controller.set(windowTopLeftPoint: NSPoint(x: screenRect.maxX + 100, y: screenRect.maxY + 100), bottomOutOfScreenAdjustmentHeight: 10)
+        let exp = expectation(description: "wait")
+        _ = XCTWaiter.wait(for: [exp], timeout: 0.2)
+        XCTAssert (controller.window?.frame.maxX ?? CGFloat.greatestFiniteMagnitude <= screenRect.maxX)
+        XCTAssert (controller.window?.frame.maxY ?? CGFloat.greatestFiniteMagnitude <= screenRect.maxY )
+    }
+
     func testReloadData() {
         let controller = VerticalCandidateController()
         let mock = Mock()