diff --git a/gfx/thebes/gfxTextRun.cpp b/gfx/thebes/gfxTextRun.cpp
index 2e2a0d239..d6cb1bdb2 100644
--- a/gfx/thebes/gfxTextRun.cpp
+++ b/gfx/thebes/gfxTextRun.cpp
@@ -1072,6 +1072,34 @@ gfxTextRun::GetAdvanceWidth(Range aRange, PropertyProvider *aProvider,
     return result + GetAdvanceForGlyphs(ligatureRange);
 }
 
+gfxFloat
+gfxTextRun::GetMinAdvanceWidth(Range aRange)
+{
+    MOZ_ASSERT(aRange.end <= GetLength(), "Substring out of range");
+
+    Range ligatureRange = aRange;
+    ShrinkToLigatureBoundaries(&ligatureRange);
+
+    gfxFloat result = std::max(
+        ComputePartialLigatureWidth(Range(aRange.start, ligatureRange.start),
+                                    nullptr),
+        ComputePartialLigatureWidth(Range(ligatureRange.end, aRange.end),
+                                    nullptr));
+
+    // XXX Do we need to take spacing into account? When each grapheme cluster
+    // takes its own line, we shouldn't be adding spacings around them.
+    gfxFloat clusterAdvance = 0;
+    for (uint32_t i = ligatureRange.start; i < ligatureRange.end; ++i) {
+        clusterAdvance += GetAdvanceForGlyph(i);
+        if (i + 1 == ligatureRange.end || IsClusterStart(i + 1)) {
+            result = std::max(result, clusterAdvance);
+            clusterAdvance = 0;
+        }
+    }
+
+    return result;
+}
+
 bool
 gfxTextRun::SetLineBreaks(Range aRange,
                           bool aLineBreakBefore, bool aLineBreakAfter,
diff --git a/gfx/thebes/gfxTextRun.h b/gfx/thebes/gfxTextRun.h
index 3c9ded1dc..a565075cd 100644
--- a/gfx/thebes/gfxTextRun.h
+++ b/gfx/thebes/gfxTextRun.h
@@ -298,6 +298,12 @@ public:
         return GetAdvanceWidth(Range(this), nullptr);
     }
 
+    /**
+     * Computes the minimum advance width for a substring assuming line
+     * breaking is allowed everywhere.
+     */
+    gfxFloat GetMinAdvanceWidth(Range aRange);
+
     /**
      * Clear all stored line breaks for the given range (both before and after),
      * and then set the line-break state before aRange.start to aBreakBefore and
diff --git a/layout/generic/nsTextFrame.cpp b/layout/generic/nsTextFrame.cpp
index e2a629a07..cb9b5da16 100644
--- a/layout/generic/nsTextFrame.cpp
+++ b/layout/generic/nsTextFrame.cpp
@@ -8244,6 +8244,16 @@ nsTextFrame::AddInlineMinISizeForFlow(nsRenderingContext *aRenderingContext,
     return;
   }
 
+  // If overflow-wrap is break-word, we can wrap everywhere.
+  if (textStyle->WordCanWrap(this)) {
+    aData->OptionallyBreak();
+    aData->mCurrentLine +=
+      textRun->GetMinAdvanceWidth(Range(start, flowEndInTextRun));
+    aData->mTrailingWhitespace = 0;
+    aData->OptionallyBreak();
+    return;
+  }
+
   AutoTArray<bool,BIG_TEXT_NODE_SIZE> hyphBuffer;
   bool *hyphBreakBefore = nullptr;
   if (hyphenating) {