diff --git a/content/base/src/nsDocumentViewer.cpp b/content/base/src/nsDocumentViewer.cpp
index 4f7de216ffb..30161f1d73a 100644
--- a/content/base/src/nsDocumentViewer.cpp
+++ b/content/base/src/nsDocumentViewer.cpp
@@ -4981,7 +4981,16 @@ NS_IMETHODIMP DocumentViewerImpl::SizeToContent()
    docShellAsItem->GetTreeOwner(getter_AddRefs(treeOwner));
    NS_ENSURE_TRUE(treeOwner, NS_ERROR_FAILURE);
 
-   NS_ENSURE_SUCCESS(treeOwner->SizeShellTo(docShellAsItem, width, height),
+   /* presContext's size was calculated in twips and has already been
+      rounded to the equivalent pixels (so the width/height calculation
+      we just performed was probably exact, though it was based on
+      values already rounded during ResizeReflow). In a surprising
+      number of instances, this rounding makes a window which for want
+      of one extra pixel's width ends up wrapping the longest line of
+      text during actual window layout. This makes the window too short,
+      generally clipping the OK/Cancel buttons. Here we add one pixel
+      to the calculated width, to circumvent this problem. */
+   NS_ENSURE_SUCCESS(treeOwner->SizeShellTo(docShellAsItem, width+1, height),
       NS_ERROR_FAILURE);
 
    return NS_OK;
diff --git a/layout/base/nsDocumentViewer.cpp b/layout/base/nsDocumentViewer.cpp
index 4f7de216ffb..30161f1d73a 100644
--- a/layout/base/nsDocumentViewer.cpp
+++ b/layout/base/nsDocumentViewer.cpp
@@ -4981,7 +4981,16 @@ NS_IMETHODIMP DocumentViewerImpl::SizeToContent()
    docShellAsItem->GetTreeOwner(getter_AddRefs(treeOwner));
    NS_ENSURE_TRUE(treeOwner, NS_ERROR_FAILURE);
 
-   NS_ENSURE_SUCCESS(treeOwner->SizeShellTo(docShellAsItem, width, height),
+   /* presContext's size was calculated in twips and has already been
+      rounded to the equivalent pixels (so the width/height calculation
+      we just performed was probably exact, though it was based on
+      values already rounded during ResizeReflow). In a surprising
+      number of instances, this rounding makes a window which for want
+      of one extra pixel's width ends up wrapping the longest line of
+      text during actual window layout. This makes the window too short,
+      generally clipping the OK/Cancel buttons. Here we add one pixel
+      to the calculated width, to circumvent this problem. */
+   NS_ENSURE_SUCCESS(treeOwner->SizeShellTo(docShellAsItem, width+1, height),
       NS_ERROR_FAILURE);
 
    return NS_OK;