Bug 1547277. If the size of the document as recorded in the frame/view tree and the document viewer diverge as a result of a call to GetContentSize make sure to invalidate when they converge again because what we draw is changing. r=mattwoodrow

The code comment mostly explains things.

Differential Revision: https://phabricator.services.mozilla.com/D29953

--HG--
extra : moz-landing-system : lando

layout/base/nsDocumentViewer.cpp

@@ -2032,6 +2032,7 @@ nsDocumentViewer::SetBoundsWithFlags(const nsIntRect& aBounds,
                                      uint32_t aFlags) {
   NS_ENSURE_TRUE(mDocument, NS_ERROR_NOT_AVAILABLE);
 
+  bool boundsChanged = mBounds.IsEqualEdges(aBounds);
   mBounds = aBounds;
 
   if (mWindow && !mAttachedToParent) {
@@ -2047,11 +2048,32 @@ nsDocumentViewer::SetBoundsWithFlags(const nsIntRect& aBounds,
     if (mPresContext->DeviceContext()->CheckDPIChange()) {
       mPresContext->UIResolutionChanged();
     }
+
     int32_t p2a = mPresContext->AppUnitsPerDevPixel();
+    nscoord width = NSIntPixelsToAppUnits(mBounds.width, p2a);
+    nscoord height = NSIntPixelsToAppUnits(mBounds.height, p2a);
+    nsView* rootView = mViewManager->GetRootView();
+    if (boundsChanged && rootView) {
+      nsRect viewDims = rootView->GetDimensions();
+      // If the view/frame tree and prescontext visible area already has the new
+      // size but we did not, then it's likely that we got reflowed in response
+      // to a call to GetContentSize. Thus there is a disconnect between the
+      // size on the document viewer/docshell/containing widget and view
+      // tree/frame tree/prescontext visible area). SetWindowDimensions compares
+      // to the root view dimenstions to determine if it needs to do anything;
+      // if they are the same as the new size it won't do anything, but we still
+      // need to invalidate because what we want to draw to the screen has
+      // changed.
+      if (viewDims.width == width && viewDims.height == height) {
+        nsIFrame* f = rootView->GetFrame();
+        if (f) {
+          f->InvalidateFrame();
+        }
+      }
+    }
+
     mViewManager->SetWindowDimensions(
-        NSIntPixelsToAppUnits(mBounds.width, p2a),
-        NSIntPixelsToAppUnits(mBounds.height, p2a),
-        !!(aFlags & nsIContentViewer::eDelayResize));
+        width, height, !!(aFlags & nsIContentViewer::eDelayResize));
   }
 
   // If there's a previous viewer, it's the one that's actually showing,