diff --git a/lib/base/rect.h b/lib/base/rect.h
index 16ac425d..58a7a25a 100644
--- a/lib/base/rect.h
+++ b/lib/base/rect.h
@@ -110,7 +110,7 @@ class RectT {
 
   bool IsSame(const RectT& other) const {
     return x0_ == other.x0_ && xsize_ == other.xsize_ && y0_ == other.y0_ &&
-           ysize_ <= other.ysize_;
+           ysize_ == other.ysize_;
   }
 
   // Returns true if this Rect fully resides in the given image. ImageT could be
diff --git a/lib/base/rect_test.cc b/lib/base/rect_test.cc
new file mode 100644
index 00000000..44af327f
--- /dev/null
+++ b/lib/base/rect_test.cc
@@ -0,0 +1,40 @@
+// Copyright (c) the JPEG XL Project Authors.
+//
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file or at
+// https://developers.google.com/open-source/licenses/bsd
+
+#include "lib/base/rect.h"
+
+#include <cstdint>
+
+#include "lib/base/testing.h"
+
+namespace jpegli {
+namespace {
+
+TEST(RectTest, IsSameRequiresEqualSize) {
+  const RectT<uint64_t> base(0, 0, 16, 16);
+  EXPECT_TRUE(base.IsSame(RectT<uint64_t>(0, 0, 16, 16)));
+  EXPECT_FALSE(base.IsSame(RectT<uint64_t>(1, 0, 16, 16)));
+  EXPECT_FALSE(base.IsSame(RectT<uint64_t>(0, 1, 16, 16)));
+  EXPECT_FALSE(base.IsSame(RectT<uint64_t>(0, 0, 8, 16)));
+  // A rect that is shorter or taller than `other` is not the same rect. The
+  // height comparison must be an equality, not `<=`.
+  EXPECT_FALSE(base.IsSame(RectT<uint64_t>(0, 0, 16, 8)));
+  EXPECT_FALSE(base.IsSame(RectT<uint64_t>(0, 0, 16, 32)));
+}
+
+// Mirrors the APNG fcTL viewport check in third_party/apngdis/dec.cc: a frame
+// whose viewport extends past the image bottom must not be treated as fully
+// inside the image, otherwise later per-row copies read out of bounds.
+TEST(RectTest, IntersectionIsSameRejectsOversizedHeight) {
+  const RectT<uint64_t> image(0, 0, 16, 16);
+  const RectT<uint64_t> inside(0, 0, 16, 16);
+  const RectT<uint64_t> too_tall(0, 0, 16, 100);
+  EXPECT_TRUE(inside.Intersection(image).IsSame(inside));
+  EXPECT_FALSE(too_tall.Intersection(image).IsSame(too_tall));
+}
+
+}  // namespace
+}  // namespace jpegli
diff --git a/lib/jpegli_lists.cmake b/lib/jpegli_lists.cmake
index 9535e26e..8edaa21b 100644
--- a/lib/jpegli_lists.cmake
+++ b/lib/jpegli_lists.cmake
@@ -241,6 +241,7 @@ set(JPEGLI_INTERNAL_TESTLIB_FILES
 )
 
 set(JPEGLI_INTERNAL_TESTS
+  base/rect_test.cc
   cms/tone_mapping_test.cc
   cms/transfer_functions_test.cc
   extras/butteraugli_test.cc