diff --git a/dom/base/Element.cpp b/dom/base/Element.cpp
index 88e72e76dc35..183ed82ba469 100644
--- a/dom/base/Element.cpp
+++ b/dom/base/Element.cpp
@@ -161,6 +161,7 @@
 #include "nsBlockFrame.h"
 
 #include "DOMMatrix.h"
+#include "MobileViewportManager.h"
 
 #ifdef ACCESSIBILITY
 #  include "nsAccessibilityService.h"
@@ -845,13 +846,17 @@ nsRect Element::GetClientAreaRect() {
     // We will always have a pres shell if we have a pres context, and we will
     // only get here if we have a pres context from the root content document
     // check
-    PresShell* presShell = doc->GetPresShell();
+    RefPtr<PresShell> presShell = doc->GetPresShell();
 
     // Ensure up to date dimensions, but don't reflow
     RefPtr<nsViewManager> viewManager = presShell->GetViewManager();
     if (viewManager) {
       viewManager->FlushDelayedResize(false);
     }
+    if (RefPtr<MobileViewportManager> mvm =
+            presShell->GetMobileViewportManager()) {
+      mvm->EnsureInitialViewportSet();
+    }
     return nsRect(nsPoint(), presContext->GetVisibleArea().Size());
   }
 
diff --git a/dom/base/nsGlobalWindowOuter.cpp b/dom/base/nsGlobalWindowOuter.cpp
index e7252c30f08a..db4ce4b1ce22 100644
--- a/dom/base/nsGlobalWindowOuter.cpp
+++ b/dom/base/nsGlobalWindowOuter.cpp
@@ -246,6 +246,7 @@
 #include "mozilla/dom/WebIDLGlobalNameHash.h"
 #include "mozilla/dom/Worklet.h"
 #include "AccessCheck.h"
+#include "MobileViewportManager.h"
 
 #ifdef HAVE_SIDEBAR
 #  include "mozilla/dom/ExternalBinding.h"
@@ -3549,7 +3550,7 @@ nsresult nsGlobalWindowOuter::GetInnerSize(CSSIntSize& aSize) {
   NS_ENSURE_STATE(mDocShell);
 
   RefPtr<nsPresContext> presContext = mDocShell->GetPresContext();
-  PresShell* presShell = mDocShell->GetPresShell();
+  RefPtr<PresShell> presShell = mDocShell->GetPresShell();
 
   if (!presContext || !presShell) {
     aSize = CSSIntSize(0, 0);
@@ -3563,6 +3564,14 @@ nsresult nsGlobalWindowOuter::GetInnerSize(CSSIntSize& aSize) {
     viewManager->FlushDelayedResize(false);
   }
 
+  // If this is called before the usual triggers for MVM to compute
+  // an initial viewport (load, DOMMetaAdded, first paint), compute
+  // one now.
+  if (RefPtr<MobileViewportManager> mvm =
+          presShell->GetMobileViewportManager()) {
+    mvm->EnsureInitialViewportSet();
+  }
+
   // FIXME: Bug 1598487 - Return the layout viewport instead of the ICB.
   nsSize viewportSize = presContext->GetVisibleArea().Size();
   if (presContext->GetDynamicToolbarState() == DynamicToolbarState::Collapsed) {
diff --git a/layout/base/MobileViewportManager.cpp b/layout/base/MobileViewportManager.cpp
index 5463c92fce1c..73c80228abc5 100644
--- a/layout/base/MobileViewportManager.cpp
+++ b/layout/base/MobileViewportManager.cpp
@@ -183,6 +183,16 @@ MobileViewportManager::Observe(nsISupports* aSubject, const char* aTopic,
   return NS_OK;
 }
 
+void MobileViewportManager::EnsureInitialViewportSet() {
+  if (!mPainted) {
+    mIsFirstPaint = true;
+    // Do not set mPainted=true because we may not have a frame
+    // tree yet and we want to make sure SetInitialViewport()
+    // gets called once we have a frame tree.
+    RefreshViewportSize(false);
+  }
+}
+
 void MobileViewportManager::SetInitialViewport() {
   MVM_LOG("%p: setting initial viewport\n", this);
   mIsFirstPaint = true;
diff --git a/layout/base/MobileViewportManager.h b/layout/base/MobileViewportManager.h
index c580e701f407..d4c7649633f7 100644
--- a/layout/base/MobileViewportManager.h
+++ b/layout/base/MobileViewportManager.h
@@ -89,6 +89,17 @@ class MobileViewportManager final : public nsIDOMEventListener,
    * updated, and the visual viewport size needs to be updated. */
   void ResolutionUpdated(mozilla::ResolutionChangeOrigin aOrigin);
 
+  /* Ensure that the initial viewport has been set based on the meta viewport
+     tag. There are two important differences between this and
+     SetInitialViewport():
+       1. This may be called before page load or before-first-paint, if
+          e.g. page content queries innerWidth early on.
+       2. This has no effect if SetInitialViewport() has already been
+          called, and thus will not clobber a viewport that has
+          previously been set (and perhaps modified since then).
+   */
+  void EnsureInitialViewportSet();
+
   /* Called to compute the initial viewport on page load or before-first-paint,
    * whichever happens first. Also called directly if we are created after the
    * presShell is initialized. */
diff --git a/testing/web-platform/meta/html/browsers/the-window-object/noopener-noreferrer-sizing.window.js.ini b/testing/web-platform/meta/html/browsers/the-window-object/noopener-noreferrer-sizing.window.js.ini
deleted file mode 100644
index 8ece66ccf7a5..000000000000
--- a/testing/web-platform/meta/html/browsers/the-window-object/noopener-noreferrer-sizing.window.js.ini
+++ /dev/null
@@ -1,9 +0,0 @@
-[noopener-noreferrer-sizing.window.html]
-  [window.open() with noreferrer should have equal viewport width and height]
-    expected:
-      if os == "android": FAIL
-
-  [window.open() with noopener should have equal viewport width and height]
-    expected:
-      if os == "android": FAIL
-