Change HarfBuzz selectionRect rounding to match simple text version

Source/platform/fonts/harfbuzz/HarfBuzzShaper.cpp

Index: Source/platform/fonts/harfbuzz/HarfBuzzShaper.cpp
diff --git a/Source/platform/fonts/harfbuzz/HarfBuzzShaper.cpp b/Source/platform/fonts/harfbuzz/HarfBuzzShaper.cpp
index b2cc9d20f4f83c29f3348055547a9aeb662be7ea..0582c1f9cedd11a489c791eefb3eed883c4ad381 100644
--- a/Source/platform/fonts/harfbuzz/HarfBuzzShaper.cpp
+++ b/Source/platform/fonts/harfbuzz/HarfBuzzShaper.cpp
@@ -34,6 +34,7 @@
 
 #include "RuntimeEnabledFeatures.h"
 #include "hb.h"
+#include "platform/LayoutUnit.h"
 #include "platform/fonts/Character.h"
 #include "platform/fonts/Font.h"
 #include "platform/fonts/harfbuzz/HarfBuzzFace.h"
@@ -1111,9 +1112,21 @@ FloatRect HarfBuzzShaper::selectionRect(const FloatPoint& point, int height, int
         toX = m_run.rtl() ? 0 : m_totalWidth;
 
     // Using floorf() and roundf() as the same as mac port.
-    if (fromX < toX)
-        return FloatRect(floorf(point.x() + fromX), point.y(), roundf(toX - fromX), height);
-    return FloatRect(floorf(point.x() + toX), point.y(), roundf(fromX - toX), height);
+    // Use LayoutUnit::epsilon() to ensure that values that cannot be stored as
+    // an integer are floored to n and not n-1 due to floating point imprecision.
+    if (fromX < toX) {
+        float pixelAlignedX = floorf(point.x() + fromX + LayoutUnit::epsilon());
+        return FloatRect(floorf(point.x() + fromX),
+            point.y(),
+            roundf(point.x() + toX) - pixelAlignedX,
+            height);
+    }
+
+    float pixelAlignedX = floorf(point.x() + toX + LayoutUnit::epsilon());
+    return FloatRect(floorf(point.x() + toX),
+        point.y(),
+        roundf(point.x() + fromX) - pixelAlignedX,
+        height);
 }
 
 } // namespace WebCore