diff --git a/src/zm_zone.cpp b/src/zm_zone.cpp
index 47e390030..ca810b75e 100644
--- a/src/zm_zone.cpp
+++ b/src/zm_zone.cpp
@@ -862,9 +862,10 @@ bool Zone::ParsePercentagePolygon(const char *poly_string, unsigned int width, u
     int32 px_x = static_cast<int32>(std::lround(pct_x * mon_w / 100.0));
     int32 px_y = static_cast<int32>(std::lround(pct_y * mon_h / 100.0));
 
-    // Clamp to monitor bounds
-    px_x = std::clamp(px_x, static_cast<int32>(0), static_cast<int32>(width));
-    px_y = std::clamp(px_y, static_cast<int32>(0), static_cast<int32>(height));
+    // Clamp to monitor bounds. Max valid pixel index is width-1/height-1;
+    // values equal to width/height cause out-of-bounds warnings in the rasterizer.
+    px_x = std::clamp(px_x, static_cast<int32>(0), static_cast<int32>(width) - 1);
+    px_y = std::clamp(px_y, static_cast<int32>(0), static_cast<int32>(height) - 1);
 
     Debug(3, "Percentage coord %.2f,%.2f -> pixel %d,%d", pct_x, pct_y, px_x, px_y);
     vertices.emplace_back(px_x, px_y);
diff --git a/tests/zm_zone.cpp b/tests/zm_zone.cpp
index a56d0cd45..42d6375e1 100644
--- a/tests/zm_zone.cpp
+++ b/tests/zm_zone.cpp
@@ -28,9 +28,9 @@ TEST_CASE("Zone::ParsePercentagePolygon: full-frame zone at 1920x1080", "[Zone]"
   REQUIRE(result == true);
   REQUIRE(polygon.GetVertices().size() == 4);
   REQUIRE(polygon.GetVertices()[0] == Vector2(0, 0));
-  REQUIRE(polygon.GetVertices()[1] == Vector2(1920, 0));
-  REQUIRE(polygon.GetVertices()[2] == Vector2(1920, 1080));
-  REQUIRE(polygon.GetVertices()[3] == Vector2(0, 1080));
+  REQUIRE(polygon.GetVertices()[1] == Vector2(1919, 0));
+  REQUIRE(polygon.GetVertices()[2] == Vector2(1919, 1079));
+  REQUIRE(polygon.GetVertices()[3] == Vector2(0, 1079));
 }
 
 TEST_CASE("Zone::ParsePercentagePolygon: center 50% zone", "[Zone]") {
@@ -68,8 +68,8 @@ TEST_CASE("Zone::ParsePercentagePolygon: different resolution", "[Zone]") {
       640, 480, polygon);
 
   REQUIRE(result == true);
-  REQUIRE(polygon.GetVertices()[1] == Vector2(640, 0));
-  REQUIRE(polygon.GetVertices()[2] == Vector2(640, 480));
+  REQUIRE(polygon.GetVertices()[1] == Vector2(639, 0));
+  REQUIRE(polygon.GetVertices()[2] == Vector2(639, 479));
 }
 
 TEST_CASE("Zone::ParsePercentagePolygon: clamping beyond 100%", "[Zone]") {
@@ -79,8 +79,8 @@ TEST_CASE("Zone::ParsePercentagePolygon: clamping beyond 100%", "[Zone]") {
       1920, 1080, polygon);
 
   REQUIRE(result == true);
-  // 110% should be clamped to monitor width
-  REQUIRE(polygon.GetVertices()[1].x_ == 1920);
+  // 110% should be clamped to max valid pixel index (width-1)
+  REQUIRE(polygon.GetVertices()[1].x_ == 1919);
 }
 
 TEST_CASE("Zone::ParsePercentagePolygon: triangle", "[Zone]") {
@@ -114,9 +114,9 @@ TEST_CASE("Zone::ParsePercentagePolygon: integer coords still work", "[Zone]") {
 
   REQUIRE(result == true);
   REQUIRE(polygon.GetVertices()[0] == Vector2(0, 0));
-  REQUIRE(polygon.GetVertices()[1] == Vector2(1920, 0));
-  REQUIRE(polygon.GetVertices()[2] == Vector2(1920, 1080));
-  REQUIRE(polygon.GetVertices()[3] == Vector2(0, 1080));
+  REQUIRE(polygon.GetVertices()[1] == Vector2(1919, 0));
+  REQUIRE(polygon.GetVertices()[2] == Vector2(1919, 1079));
+  REQUIRE(polygon.GetVertices()[3] == Vector2(0, 1079));
 }
 
 TEST_CASE("Zone::ParsePolygonString: basic pixel parsing", "[Zone]") {
@@ -166,9 +166,9 @@ TEST_CASE("Zone::ParsePercentagePolygon: percentage to pixel conversion", "[Zone
     auto const &verts = polygon.GetVertices();
     REQUIRE(verts.size() == 4);
     REQUIRE(verts[0] == Vector2(0, 0));
-    REQUIRE(verts[1] == Vector2(1920, 0));
-    REQUIRE(verts[2] == Vector2(1920, 1080));
-    REQUIRE(verts[3] == Vector2(0, 1080));
+    REQUIRE(verts[1] == Vector2(1919, 0));
+    REQUIRE(verts[2] == Vector2(1919, 1079));
+    REQUIRE(verts[3] == Vector2(0, 1079));
   }
 
   SECTION("50% rectangle converts to half-resolution pixels") {
@@ -184,16 +184,16 @@ TEST_CASE("Zone::ParsePercentagePolygon: percentage to pixel conversion", "[Zone
   }
 
   SECTION("values are clamped to monitor bounds") {
-    // 100% should clamp to exact monitor dimensions
+    // 100% should clamp to max valid pixel index (width-1, height-1)
     bool ok = Zone::ParsePercentagePolygon("0,0 100,0 100,100 0,100", width, height, polygon);
     REQUIRE(ok);
 
     auto const &verts = polygon.GetVertices();
     for (auto const &v : verts) {
       REQUIRE(v.x_ >= 0);
-      REQUIRE(v.x_ <= static_cast<int>(width));
+      REQUIRE(v.x_ < static_cast<int>(width));
       REQUIRE(v.y_ >= 0);
-      REQUIRE(v.y_ <= static_cast<int>(height));
+      REQUIRE(v.y_ < static_cast<int>(height));
     }
   }
 }
@@ -213,12 +213,12 @@ TEST_CASE("Zone: pixel values through ParsePercentagePolygon produce wrong resul
   auto const &verts = polygon.GetVertices();
   REQUIRE(verts.size() == 4);
 
-  // 639% of 1920 = 12268.8 -> clamped to 1920
-  // 479% of 1080 = 5173.2 -> clamped to 1080
+  // 639% of 1920 = 12268.8 -> clamped to width-1
+  // 479% of 1080 = 5173.2 -> clamped to height-1
   // All non-zero coords get clamped to monitor bounds — the zone is
   // degenerate (covers the full monitor instead of a sub-region)
-  REQUIRE(verts[1] == Vector2(static_cast<int>(width), 0));
-  REQUIRE(verts[2] == Vector2(static_cast<int>(width), static_cast<int>(height)));
+  REQUIRE(verts[1] == Vector2(static_cast<int>(width) - 1, 0));
+  REQUIRE(verts[2] == Vector2(static_cast<int>(width) - 1, static_cast<int>(height) - 1));
 }
 
 // --- Auto-detect format tests ---