diff --git a/mozilla/layout/generic/nsLineLayout.cpp b/mozilla/layout/generic/nsLineLayout.cpp
index 68375601dcd..935e93d9758 100644
--- a/mozilla/layout/generic/nsLineLayout.cpp
+++ b/mozilla/layout/generic/nsLineLayout.cpp
@@ -1267,21 +1267,29 @@ nsLineLayout::CanPlaceFrame(PerFrameData* pfd,
  }
 
   if (aFrameCanContinueTextRun) {
-    // Let it fit, but we reserve the right to roll back
-    // to before the text run! Note that we usually won't get here because
-    // a text frame will break itself to avoid exceeding the available width.
+    // Let it fit, but we reserve the right to roll back.
+    // Note that we usually won't get here because a text frame will break
+    // itself to avoid exceeding the available width.
     // We'll only get here for text frames that couldn't break early enough.
 #ifdef NOISY_CAN_PLACE_FRAME
     printf("   ==> placing overflowing textrun, requesting backup\n");
 #endif
+
+    // We will want to try backup.
+    SetFlag(LL_NEEDBACKUP, PR_TRUE);
+
     if (!aCanRollBackBeforeFrame) {
       // Nowhere to roll back to, so make this fit
       return PR_TRUE;
     }
-
-    // We have something to roll back to. So, signal that we will to roll back,
-    // and fall through to not place this frame.
-    SetFlag(LL_NEEDBACKUP, PR_TRUE);
+    if (pfd->mSpan) {
+      // Allow spans to fit here. We don't want a span to fail to fit just
+      // because one of its children didn't fit; there may be a break opportunity
+      // we can roll back to inside the span.
+      return PR_TRUE;
+    }
+    // There is a break opportunity before the frame, so we can stop line
+    // reflow now.
   }
 
 #ifdef NOISY_CAN_PLACE_FRAME
diff --git a/mozilla/layout/reftests/bugs/402950-1-ref.html b/mozilla/layout/reftests/bugs/402950-1-ref.html
new file mode 100644
index 00000000000..40911b5e6cf
--- /dev/null
+++ b/mozilla/layout/reftests/bugs/402950-1-ref.html
@@ -0,0 +1,15 @@
+<!DOCTYPE HTML>
+<html><head>
+<style>
+p { width: 13em; border: 1px solid black; }
+p { white-space: nowrap; }
+a { color:pink; }
+</style>
+</head>
+<body>
+<p><span>mmmmmmmmmm</span><br>
+<span>mmmmm<a>mmmmm</a></span></p>
+<p>m <span>mmmmmmmmmm</span><br>
+<span>m<a>mmmmmmmmm</a></span></p>
+</body>
+</html>
diff --git a/mozilla/layout/reftests/bugs/402950-1.html b/mozilla/layout/reftests/bugs/402950-1.html
new file mode 100644
index 00000000000..923678d66ee
--- /dev/null
+++ b/mozilla/layout/reftests/bugs/402950-1.html
@@ -0,0 +1,21 @@
+<!DOCTYPE HTML>
+<html><head>
+<style>
+p { width: 13em; border: 1px solid black; }
+p span { white-space: nowrap; }
+a { color:pink; }
+</style>
+</head>
+<body>
+<!-- We should be able to break even when the only break opportunity is within a span
+that overflows the line width (the break opportunity after the whitespace is treated
+as a break before the first character of the second span) -->
+<p><span>mmmmmmmmmm</span>
+<span>mmmmm<a>mmmmm</a></span></p>
+<!-- A span that overflows the line width should be allowed to fit, it might have a break
+opportunity inside it (the break opportunity after the whitespace is treated
+as a break before the first character of the second span) -->
+<p>m <span>mmmmmmmmmm</span>
+<span>m<a>mmmmmmmmm</a></span></p>
+</body>
+</html>
diff --git a/mozilla/layout/reftests/bugs/reftest.list b/mozilla/layout/reftests/bugs/reftest.list
index 49100e017f5..e5b4dc1be6c 100644
--- a/mozilla/layout/reftests/bugs/reftest.list
+++ b/mozilla/layout/reftests/bugs/reftest.list
@@ -450,6 +450,7 @@ fails == 386310-1d.html 386310-1-ref.html
 == 400171-2c.html 400171-2-ref.html
 == 400421-1.html 400421-1-ref.html
 == 400826-1.html 400826-1-ref.html
+== 402950-1.html 402950-1-ref.html
 == 403129-1.html 403129-1-ref.html
 == 403129-2.html 403129-2-ref.html
 == 403129-3.html 403129-3-ref.html
