Bug 1742006 - Fix documents of rect types' Union and SaturatingUnion, and add a test. r=gfx-reviewers,mstange

The documents of Union and SaturatingUnion promise that |this| will be return
when both rects are empty, but the current implementation returns aRect instead.

This patch fixes the documents and added a test case.

Differential Revision: https://phabricator.services.mozilla.com/D131568

gfx/2d/BaseRect.h

@@ -174,9 +174,8 @@ struct BaseRect {
   }
 
   // Returns the smallest rectangle that contains both the area of both
-  // this and aRect2.
-  // Thus, empty input rectangles are ignored.
-  // If both rectangles are empty, returns this.
+  // this and aRect. Thus, empty input rectangles are ignored.
+  // Note: if both rectangles are empty, returns aRect.
   // WARNING! This is not safe against overflow, prefer using SafeUnion instead
   // when dealing with int-based rects.
   [[nodiscard]] Sub Union(const Sub& aRect) const {

gfx/2d/RectAbsolute.h

@@ -142,9 +142,8 @@ struct BaseRectAbsolute {
   void SetEmpty() { left = right = top = bottom = 0; }
 
   // Returns the smallest rectangle that contains both the area of both
-  // this and aRect2.
-  // Thus, empty input rectangles are ignored.
-  // If both rectangles are empty, returns this.
+  // this and aRect. Thus, empty input rectangles are ignored.
+  // Note: if both rectangles are empty, returns aRect.
   // WARNING! This is not safe against overflow, prefer using SafeUnion instead
   // when dealing with int-based rects.
   [[nodiscard]] Sub Union(const Sub& aRect) const {

gfx/src/nsRect.h

@@ -54,6 +54,9 @@ struct nsRect : public mozilla::gfx::BaseRect<nscoord, nsRect, nsPoint, nsSize,
   // overflowing nscoord values in the 'width' and 'height' fields by
   // clamping the width and height values to nscoord_MAX if necessary.
 
+  // Returns the smallest rectangle that contains both the area of both
+  // this and aRect. Thus, empty input rectangles are ignored.
+  // Note: if both rectangles are empty, returns aRect.
   [[nodiscard]] nsRect SaturatingUnion(const nsRect& aRect) const {
     if (IsEmpty()) {
       return aRect;

gfx/tests/gtest/TestRect.cpp

@@ -9,6 +9,7 @@
 
 #include "gfxTypes.h"
 #include "nsRect.h"
+#include "nsRectAbsolute.h"
 #include "gfxRect.h"
 #include "mozilla/gfx/Rect.h"
 #include "mozilla/gfx/RectAbsolute.h"
@@ -435,6 +436,18 @@ static bool TestUnion() {
   return true;
 }
 
+template <class RectType>
+static void TestUnionEmptyRects() {
+  RectType rect1(10, 10, 0, 50);
+  RectType rect2(5, 5, 40, 0);
+  EXPECT_TRUE(rect1.IsEmpty() && rect2.IsEmpty());
+
+  RectType dest = rect1.Union(rect2);
+  EXPECT_TRUE(dest.IsEmpty() && dest.IsEqualEdges(rect2))
+      << "Test the case where both rects are empty, and the result is the "
+         "same value passing into Union()";
+}
+
 static bool TestFiniteGfx() {
   float posInf = std::numeric_limits<float>::infinity();
   float negInf = -std::numeric_limits<float>::infinity();
@@ -578,6 +591,7 @@ TEST(Gfx, nsRect)
   TestIntersects<nsRect>();
   TestIntersection<nsRect>();
   TestUnion<nsRect>();
+  TestUnionEmptyRects<nsRect>();
   TestBug1135677<nsRect>();
   TestSetWH<nsRect>();
   TestSwap<nsRect>();
@@ -591,6 +605,7 @@ TEST(Gfx, nsIntRect)
   TestIntersects<nsIntRect>();
   TestIntersection<nsIntRect>();
   TestUnion<nsIntRect>();
+  TestUnionEmptyRects<nsIntRect>();
   TestBug1135677<nsIntRect>();
   TestSetWH<nsIntRect>();
   TestSwap<nsIntRect>();
@@ -604,12 +619,19 @@ TEST(Gfx, gfxRect)
   TestIntersects<gfxRect>();
   TestIntersection<gfxRect>();
   TestUnion<gfxRect>();
+  TestUnionEmptyRects<gfxRect>();
   TestBug1135677<gfxRect>();
   TestFiniteGfx();
   TestSetWH<gfxRect>();
   TestSwap<gfxRect>();
 }
 
+TEST(Gfx, nsRectAbsolute)
+{ TestUnionEmptyRects<nsRectAbsolute>(); }
+
+TEST(Gfx, IntRectAbsolute)
+{ TestUnionEmptyRects<IntRectAbsolute>(); }
+
 static void TestMoveInsideAndClamp(IntRect aSrc, IntRect aTarget,
                                    IntRect aExpected) {
   // Test the implementation in BaseRect (x/y/width/height representation)