Don't rely exclusively on scroll events to determine content resolution.

Default to 1 rather 0 so there's a reasonable default in the abscense of
any events and update the resolution when the scroll area is update as
scroll events are not always received (or accurate it seems but those
come later and scroll area updates are infrequent so this doesn't help
with that).

src/qmozview_p.cpp

@@ -809,6 +809,17 @@ void QMozViewPrivate::OnFirstPaint(int32_t aX, int32_t aY)
 
 void QMozViewPrivate::OnScrolledAreaChanged(unsigned int aWidth, unsigned int aHeight)
 {
+    // Normally these come from HandleScrollEvent but for some documents no such event is generated.
+
+    const float contentResoution = contentWindowSize(mMozWindow).width() / aWidth;
+    if (qFuzzyIsNull(mContentResolution) && !qFuzzyIsNull(contentResoution)) {
+        mContentResolution = contentResoution;
+    }
+
+    if (mContentRect.isEmpty()) {
+        mContentRect.setSize(QSizeF(aWidth, aHeight));
+    }
+
     UpdateScrollArea(aWidth * mContentResolution, aHeight * mContentResolution,
                      mScrollableOffset.x(), mScrollableOffset.y());
 }
@@ -898,17 +909,23 @@ bool QMozViewPrivate::HandleScrollEvent(bool aIsRootScrollFrame, const gfxRect &
     if (!aIsRootScrollFrame)
         return false;
 
-    mContentResolution = contentWindowSize(mMozWindow).width() / aContentRect.width;
-
     if (mContentRect.x() != aContentRect.x || mContentRect.y() != aContentRect.y ||
             mContentRect.width() != aContentRect.width ||
             mContentRect.height() != aContentRect.height) {
         mContentRect.setRect(aContentRect.x, aContentRect.y, aContentRect.width, aContentRect.height);
         mViewIface->viewAreaChanged();
     }
 
-    UpdateScrollArea(aScrollableSize.width * mContentResolution, aScrollableSize.height * mContentResolution,
-                     aContentRect.x * mContentResolution, aContentRect.y * mContentResolution);
+    float contentResoution = contentWindowSize(mMozWindow).width() / aContentRect.width;
+    if (!qFuzzyIsNull(contentResoution)) {
+        mContentResolution = contentResoution;
+        UpdateScrollArea(
+                    aScrollableSize.width * mContentResolution,
+                    aScrollableSize.height * mContentResolution,
+                    aContentRect.x * mContentResolution,
+                    aContentRect.y * mContentResolution);
+    }
+
     return false;
 }