Bug 1509552 - Ensure MobileViewportManager doesn't choose a zoom level that makes the content smaller than the visual viewport. r=kats

Differential Revision: https://phabricator.services.mozilla.com/D13315

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

layout/base/MobileViewportManager.cpp

@@ -324,9 +324,22 @@ MobileViewportManager::UpdateResolution(const nsViewportInfo& aViewportInfo,
   } else {  // aType == UpdateType::ContentSize
     MOZ_ASSERT(aType == UpdateType::ContentSize);
     MOZ_ASSERT(aDisplayWidthChangeRatio.isNothing());
+
+    // We try to scale down the contents only IF the document has no initial-scale
+    // AND IF it's the initial paint AND IF it's not restored documents.
+    if (mIsFirstPaint && !mRestoreResolution &&
+        !aViewportInfo.IsDefaultZoomValid()) {
       if (zoom != intrinsicScale) {
         newZoom = Some(intrinsicScale);
       }
+    } else {
+      // Even in other scenarios, we want to ensure that zoom level is
+      // not _smaller_ than the intrinsic scale, otherwise we might be
+      // trying to show regions where there is no content to show.
+      if (zoom < intrinsicScale) {
+        newZoom = Some(intrinsicScale);
+      }
+    }
   }
 
   // If the zoom has changed, update the pres shell resolution and the
@@ -516,13 +529,6 @@ MobileViewportManager::ShrinkToDisplaySizeIfNeeded(
     return;
   }
 
-  // We try to scale down the contents only IF the document has no initial-scale
-  // AND IF it's the initial paint AND IF it's not restored documents.
-  if (aViewportInfo.IsDefaultZoomValid() ||
-      !mIsFirstPaint || mRestoreResolution) {
-    return;
-  }
-
   nsIScrollableFrame* rootScrollableFrame =
     mPresShell->GetRootScrollFrameAsScrollable();
   if (rootScrollableFrame) {