diff --git a/layout/base/nsLayoutUtils.cpp b/layout/base/nsLayoutUtils.cpp
index db594605273..5a89f12c8dd 100644
--- a/layout/base/nsLayoutUtils.cpp
+++ b/layout/base/nsLayoutUtils.cpp
@@ -4765,14 +4765,6 @@ nsLayoutUtils::FontSizeInflationInner(const nsIFrame *aFrame,
   return (1.0f / ratio) + (1.0f / 3.0f);
 }
 
-static inline bool
-InflationDataSaysEnabled(const nsIFrame *aFrame)
-{
-  nsFontInflationData *data =
-    nsFontInflationData::FindFontInflationDataFor(aFrame);
-  return data && data->InflationEnabled();
-}
-
 static bool
 ShouldInflateFontsForContainer(const nsIFrame *aFrame)
 {
@@ -4789,55 +4781,35 @@ ShouldInflateFontsForContainer(const nsIFrame *aFrame)
          !(aFrame->GetStateBits() & NS_FRAME_IN_CONSTRAINED_HEIGHT) &&
          // We also want to disable font inflation for containers that have
          // preformatted text.
-         styleText->WhiteSpaceCanWrap() &&
-         InflationDataSaysEnabled(aFrame);
+         styleText->WhiteSpaceCanWrap();
 }
 
 nscoord
 nsLayoutUtils::InflationMinFontSizeFor(const nsIFrame *aFrame,
                                        WidthDetermination aWidthDetermination)
 {
-#ifdef DEBUG
-  if (aWidthDetermination == eNotInReflow) {
-    // Check that neither this frame nor any of its ancestors are
-    // currently being reflowed.
-    // It's ok for box frames (but not arbitrary ancestors of box frames)
-    // since they set their size before reflow.
-    if (!(aFrame->IsBoxFrame() && IsContainerForFontSizeInflation(aFrame))) {
-      for (const nsIFrame *f = aFrame; f; f = f->GetParent()) {
-        NS_ABORT_IF_FALSE(!(f->GetStateBits() & NS_FRAME_IN_REFLOW),
-                          "must call nsHTMLReflowState& version during reflow");
-      }
-    }
-    // It's ok if frames are dirty, or even if they've never been
-    // reflowed, since they will be eventually and then we'll get the
-    // right size.
-  }
-#endif
-
   nsPresContext *presContext = aFrame->PresContext();
   if (!FontSizeInflationEnabled(presContext) ||
       presContext->mInflationDisabledForShrinkWrap) {
     return 0;
   }
 
-  if (aWidthDetermination == eInReflow) {
-    nsIFrame *container = presContext->mCurrentInflationContainer;
-    if (!container || !ShouldInflateFontsForContainer(container)) {
-      return 0;
-    }
-    return MinimumFontSizeFor(presContext,
-                              presContext->mCurrentInflationContainerWidth);
-  }
-
   for (const nsIFrame *f = aFrame; f; f = f->GetParent()) {
     if (IsContainerForFontSizeInflation(f)) {
       if (!ShouldInflateFontsForContainer(f)) {
         return 0;
       }
 
+      nsFontInflationData *data =
+        nsFontInflationData::FindFontInflationDataFor(aFrame);
+      // FIXME: The need to null-check here is sort of a bug, and might
+      // lead to incorrect results.
+      if (!data || !data->InflationEnabled()) {
+        return 0;
+      }
+
       return MinimumFontSizeFor(aFrame->PresContext(),
-                                f->GetContentRect().width);
+                                data->EffectiveWidth());
     }
   }
 
@@ -4850,24 +4822,6 @@ float
 nsLayoutUtils::FontSizeInflationFor(const nsIFrame *aFrame,
                                     WidthDetermination aWidthDetermination)
 {
-#ifdef DEBUG
-  if (aWidthDetermination == eNotInReflow) {
-    // Check that neither this frame nor any of its ancestors are
-    // currently being reflowed.
-    // It's ok for box frames (but not arbitrary ancestors of box frames)
-    // since they set their size before reflow.
-    if (!(aFrame->IsBoxFrame() && IsContainerForFontSizeInflation(aFrame))) {
-      for (const nsIFrame *f = aFrame; f; f = f->GetParent()) {
-        NS_ABORT_IF_FALSE(!(f->GetStateBits() & NS_FRAME_IN_REFLOW),
-                          "must call nsHTMLReflowState& version during reflow");
-      }
-    }
-    // It's ok if frames are dirty, or even if they've never been
-    // reflowed, since they will be eventually and then we'll get the
-    // right size.
-  }
-#endif
-
   if (!FontSizeInflationEnabled(aFrame->PresContext())) {
     return 1.0;
   }
diff --git a/layout/reftests/font-inflation/reftest.list b/layout/reftests/font-inflation/reftest.list
index 4976dc7530d..12ba5350007 100644
--- a/layout/reftests/font-inflation/reftest.list
+++ b/layout/reftests/font-inflation/reftest.list
@@ -92,3 +92,5 @@ fuzzy-if(gtk2Widget,1,8) test-pref(font.size.inflation.emPerLine,15) test-pref(f
 fuzzy-if(gtk2Widget,1,8) test-pref(font.size.inflation.emPerLine,15) test-pref(font.size.inflation.lineThreshold,100) == threshold-select-combobox-contents-under-2.html threshold-select-combobox-contents-under-2.html
 fuzzy-if(gtk2Widget,1,8) test-pref(font.size.inflation.emPerLine,15) test-pref(font.size.inflation.lineThreshold,100) == threshold-select-combobox-contents-at-1.html threshold-select-combobox-contents-at-1-ref.html
 fuzzy-if(gtk2Widget,1,8) test-pref(font.size.inflation.emPerLine,15) test-pref(font.size.inflation.lineThreshold,100) == threshold-select-combobox-contents-at-2.html threshold-select-combobox-contents-at-2-ref.html
+
+test-pref(font.size.inflation.emPerLine,15) test-pref(font.size.inflation.lineThreshold,0) == relevant-width-1.html relevant-width-1-ref.html
diff --git a/layout/reftests/font-inflation/relevant-width-1-ref.html b/layout/reftests/font-inflation/relevant-width-1-ref.html
new file mode 100644
index 00000000000..90b67e1b6ad
--- /dev/null
+++ b/layout/reftests/font-inflation/relevant-width-1-ref.html
@@ -0,0 +1,9 @@
+<!DOCTYPE HTML>
+<style>
+div { font-size: 34px; width: 450px }
+div.inner { width: 300px }
+</style>
+<div class="middle">
+  Text in middle.
+  <div class="inner">Text in inner.</div>
+</div>
diff --git a/layout/reftests/font-inflation/relevant-width-1.html b/layout/reftests/font-inflation/relevant-width-1.html
new file mode 100644
index 00000000000..814917786d8
--- /dev/null
+++ b/layout/reftests/font-inflation/relevant-width-1.html
@@ -0,0 +1,16 @@
+<!DOCTYPE HTML>
+<style>
+div.outer { font-size: 12px; width: 600px }
+div.middle { width: 450px; }
+div.inner { width: 300px; }
+</style>
+<!--
+In a 450px container, the minimum font size at 15em per line is 30px.
+This means we map 0px-45px into 30px-45px, so 12px gets mapped to 34px.
+-->
+<div class="outer">
+  <div class="middle">
+    Text in middle.
+    <div class="inner">Text in inner.</div>
+  </div>
+</div>