Bug 696746, round width rather than adding one to prevent an extra reflow, r=roc

layout/base/nsDocumentViewer.cpp

@@ -3272,23 +3272,19 @@ NS_IMETHODIMP DocumentViewerImpl::SizeToContent()
    NS_ENSURE_TRUE(shellArea.width != NS_UNCONSTRAINEDSIZE &&
                   shellArea.height != NS_UNCONSTRAINEDSIZE,
                   NS_ERROR_FAILURE);
-   width = presContext->AppUnitsToDevPixels(shellArea.width);
+
+   // Sometimes wrapped lines can create a situation where the width is a
+   // non-integer. Rather than figure out why now, we just round the width up
+   // to take this extra width into account. This makes the shell one pixel
+   // wider and causes an extra reflow.
+   width = NSToIntRoundUp(float(shellArea.width) / mDeviceContext->AppUnitsPerDevPixel());
    height = presContext->AppUnitsToDevPixels(shellArea.height);
 
    nsCOMPtr<nsIDocShellTreeOwner> treeOwner;
    docShellAsItem->GetTreeOwner(getter_AddRefs(treeOwner));
    NS_ENSURE_TRUE(treeOwner, NS_ERROR_FAILURE);
 
-   /* presContext's size was calculated in app units 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_ENSURE_SUCCESS(treeOwner->SizeShellTo(docShellAsItem, width, height),
       NS_ERROR_FAILURE);
 
    return NS_OK;