diff --git a/Lib/v2/WindowManager.ahk b/Lib/v2/WindowManager.ahk
index f35d566..8611fa2 100644
--- a/Lib/v2/WindowManager.ahk
+++ b/Lib/v2/WindowManager.ahk
@@ -47,12 +47,13 @@ WaitForProcess(processName, timeout := 30) {
 }
 
 GetMonitorAtPos(x, y, api := "") {
-    if !api
-        api := SystemWindowAPI()
-
-    count := api.MonitorGetCount()
+    count := IsObject(api) ? api.MonitorGetCount() : MonitorGetCount()
     Loop count {
-        api.MonitorGet(A_Index, &l, &t, &r, &b)
+        if IsObject(api)
+            api.MonitorGet(A_Index, &l, &t, &r, &b)
+        else
+            MonitorGet(A_Index, &l, &t, &r, &b)
+
         if (l <= x && x <= r && t <= y && y <= b)
             return A_Index
     }
diff --git a/tests/WindowManager_Test.ahk b/tests/WindowManager_Test.ahk
new file mode 100644
index 0000000..51b0af0
--- /dev/null
+++ b/tests/WindowManager_Test.ahk
@@ -0,0 +1,102 @@
+#Requires AutoHotkey v2.0
+#SingleInstance Force
+
+#Include %A_ScriptDir%\..\Lib\v2\AHK_Common.ahk
+#Include %A_ScriptDir%\..\Lib\v2\WindowManager.ahk
+
+InitScript(false, false, false) ; disable UIA, Admin, and optimization for the test
+
+; Setup testing output
+stdout := FileOpen("*", "w `n")
+testsPassed := 0
+testsFailed := 0
+
+AssertEqual(expected, actual, context) {
+    global testsPassed, testsFailed, stdout
+    if (expected == actual) {
+        testsPassed++
+        stdout.WriteLine("PASS: " . context)
+    } else {
+        testsFailed++
+        stdout.WriteLine("FAIL: " . context . " - Expected '" . expected . "', but got '" . actual . "'")
+    }
+}
+
+class MockSystemWindowAPI {
+    __New(monitors) {
+        this.monitors := monitors
+    }
+
+    MonitorGetCount() {
+        return this.monitors.Length
+    }
+
+    MonitorGet(N, &Left, &Top, &Right, &Bottom) {
+        if (N < 1 || N > this.monitors.Length)
+            return false
+        mon := this.monitors[N]
+        Left := mon.l
+        Top := mon.t
+        Right := mon.r
+        Bottom := mon.b
+        return true
+    }
+}
+
+try {
+    stdout.WriteLine("Starting GetMonitorAtPos tests...")
+
+    ; Test Set 1: Single Monitor (0,0 to 1920,1080)
+    mon1 := {l: 0, t: 0, r: 1920, b: 1080}
+    apiSingle := MockSystemWindowAPI([mon1])
+
+    AssertEqual(1, GetMonitorAtPos(960, 540, apiSingle), "Single monitor - Center")
+    AssertEqual(1, GetMonitorAtPos(0, 0, apiSingle), "Single monitor - Top Left edge")
+    AssertEqual(1, GetMonitorAtPos(1920, 1080, apiSingle), "Single monitor - Bottom Right edge")
+    AssertEqual(-1, GetMonitorAtPos(-1, 540, apiSingle), "Single monitor - Out of bounds Left")
+    AssertEqual(-1, GetMonitorAtPos(1921, 540, apiSingle), "Single monitor - Out of bounds Right")
+    AssertEqual(-1, GetMonitorAtPos(960, -1, apiSingle), "Single monitor - Out of bounds Top")
+    AssertEqual(-1, GetMonitorAtPos(960, 1081, apiSingle), "Single monitor - Out of bounds Bottom")
+
+    ; Test Set 2: Multiple Monitors
+    ; Mon 1: Primary 1920x1080 (0,0 to 1920,1080)
+    ; Mon 2: Left 1080x1920 (-1080,0 to 0,1920)
+    ; Mon 3: Right 1920x1080 (1920,0 to 3840,1080)
+    ; Mon 4: Top 1920x1080 (0,-1080 to 1920,0)
+    mon_multi_1 := {l: 0, t: 0, r: 1920, b: 1080}
+    mon_multi_2 := {l: -1080, t: 0, r: 0, b: 1920}
+    mon_multi_3 := {l: 1920, t: 0, r: 3840, b: 1080}
+    mon_multi_4 := {l: 0, t: -1080, r: 1920, b: 0}
+    apiMulti := MockSystemWindowAPI([mon_multi_1, mon_multi_2, mon_multi_3, mon_multi_4])
+
+    AssertEqual(1, GetMonitorAtPos(960, 540, apiMulti), "Multi monitor - Primary Center")
+    AssertEqual(2, GetMonitorAtPos(-500, 500, apiMulti), "Multi monitor - Left Monitor")
+    AssertEqual(3, GetMonitorAtPos(2500, 500, apiMulti), "Multi monitor - Right Monitor")
+    AssertEqual(4, GetMonitorAtPos(960, -500, apiMulti), "Multi monitor - Top Monitor")
+
+    ; Edge case overlapping bounds (falls to first match)
+    AssertEqual(1, GetMonitorAtPos(0, 0, apiMulti), "Multi monitor - Origin overlapping")
+    AssertEqual(1, GetMonitorAtPos(1920, 500, apiMulti), "Multi monitor - Border overlapping Primary/Right")
+
+    ; Out of bounds in multi-monitor setup
+    AssertEqual(-1, GetMonitorAtPos(960, 2000, apiMulti), "Multi monitor - Out of bounds Bottom")
+    AssertEqual(-1, GetMonitorAtPos(-2000, 500, apiMulti), "Multi monitor - Out of bounds Far Left")
+
+    ; Test Set 3: Zero Monitors
+    apiZero := MockSystemWindowAPI([])
+    AssertEqual(-1, GetMonitorAtPos(0, 0, apiZero), "Zero monitors - Should return -1")
+
+    ; Final Results
+    stdout.WriteLine("---")
+    stdout.WriteLine("Tests Passed: " . testsPassed)
+    stdout.WriteLine("Tests Failed: " . testsFailed)
+
+} finally {
+    ; No teardown needed
+}
+
+if (testsFailed > 0) {
+    ExitApp(1)
+}
+
+ExitApp(0)