diff --git a/layout/reftests/bugs/615121-1-ref.html b/layout/reftests/bugs/615121-1-ref.html
new file mode 100644
index 00000000000..c1993abcfb1
--- /dev/null
+++ b/layout/reftests/bugs/615121-1-ref.html
@@ -0,0 +1,22 @@
+<html>
+<head>
+<style type="text/css">
+@font-face {
+  font-family: schz;
+  src: url(../fonts/sil/ScheherazadeRegOT.ttf);
+}
+div {
+  font-family: schz;
+  font-size: 32px;
+  margin: 32px;
+}
+</style>
+</head>
+<body>
+<div>
+<p>The letter and diacritic should be hidden by the black box:<br>
+<span style="background:black;padding:0 .5em;margin-right:-1em;">
+</span>&nbsp;
+</div>
+</body>
+</html>
diff --git a/layout/reftests/bugs/615121-1.html b/layout/reftests/bugs/615121-1.html
new file mode 100644
index 00000000000..93e0937af6f
--- /dev/null
+++ b/layout/reftests/bugs/615121-1.html
@@ -0,0 +1,24 @@
+<html>
+<head>
+<style type="text/css">
+@font-face {
+  font-family: schz;
+  src: url(../fonts/sil/ScheherazadeRegOT.ttf);
+}
+div {
+  font-family: schz;
+  font-size: 32px;
+  margin: 32px;
+}
+</style>
+</head>
+<body>
+<!-- test for bug 615121: flipped sign for diacritic y-offset
+     will make it project above the em-height box -->
+<div>
+<p>The letter and diacritic should be hidden by the black box:<br>
+<span style="background:black;padding:0 .5em;margin-right:-1em;">
+</span>&nbsp;&#x628;&#x651;&#x64c;&nbsp;
+</div>
+</body>
+</html>
diff --git a/layout/reftests/bugs/615121-2-notref.html b/layout/reftests/bugs/615121-2-notref.html
new file mode 100644
index 00000000000..af23943c7cc
--- /dev/null
+++ b/layout/reftests/bugs/615121-2-notref.html
@@ -0,0 +1,22 @@
+<html>
+<head>
+<style type="text/css">
+@font-face {
+  font-family: schz;
+  src: url(../fonts/sil/ScheherazadeRegOT.ttf);
+}
+div {
+  font-family: schz;
+  font-size: 32px;
+  margin: 32px;
+}
+</style>
+</head>
+<body>
+<div>
+<p>Diacritics should stack above the black box:<br>
+<span style="background:black;padding:0 .5em;margin-right:-1em;">
+</span>&nbsp;
+</div>
+</body>
+</html>
diff --git a/layout/reftests/bugs/615121-2.html b/layout/reftests/bugs/615121-2.html
new file mode 100644
index 00000000000..7fb9e3791f4
--- /dev/null
+++ b/layout/reftests/bugs/615121-2.html
@@ -0,0 +1,24 @@
+<html>
+<head>
+<style type="text/css">
+@font-face {
+  font-family: schz;
+  src: url(../fonts/sil/ScheherazadeRegOT.ttf);
+}
+div {
+  font-family: schz;
+  font-size: 32px;
+  margin: 32px;
+}
+</style>
+</head>
+<body>
+<!-- test for bug 615121: diacritics should stack to project above the em-height;
+     sign error will make them overprint the base letter instead -->
+<div>
+<p>Diacritics should stack above the black box:<br>
+<span style="background:black;padding:0 .5em;margin-right:-1em;">
+</span>&nbsp;&#x679;&#x651;&#x64c;&#x610;&nbsp;
+</div>
+</body>
+</html>
diff --git a/layout/reftests/bugs/reftest.list b/layout/reftests/bugs/reftest.list
index 3ff8ab9b479..fb784763972 100644
--- a/layout/reftests/bugs/reftest.list
+++ b/layout/reftests/bugs/reftest.list
@@ -1551,3 +1551,5 @@ fails-if(!haveTestPlugin) == 599476.html 599476-ref.html
 == 613433-3.html 613433-1-ref.html
 == 613433-3.html 613433-2-ref.html
 == 613433-3.html 613433-3-ref.html
+HTTP(..) == 615121-1.html 615121-1-ref.html
+HTTP(..) != 615121-2.html 615121-2-notref.html
