diff --git a/dom/html/test/browser.ini b/dom/html/test/browser.ini
index 45b045453fab..cb02f7f551b2 100644
--- a/dom/html/test/browser.ini
+++ b/dom/html/test/browser.ini
@@ -26,6 +26,7 @@ skip-if = bits == 64 && (os == "mac" || os == "linux") #Bug 1646862
 [browser_containerLoadingContent.js]
 skip-if =
   os == "win" && os_version == "6.1" # Skip on Azure - frequent failure
+[browser_ImageDocument_svg_zoom.js]
 [browser_submission_flush.js]
 [browser_refresh_after_document_write.js]
 support-files =
diff --git a/dom/html/test/browser_ImageDocument_svg_zoom.js b/dom/html/test/browser_ImageDocument_svg_zoom.js
new file mode 100644
index 000000000000..2c22ad4dd8ce
--- /dev/null
+++ b/dom/html/test/browser_ImageDocument_svg_zoom.js
@@ -0,0 +1,38 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const URL = `data:image/svg+xml,<svg viewBox="0 0 100 100" xmlns="http://www.w3.org/2000/svg"><rect width="100" height="100" fill="green"/></svg>`;
+
+function test_once() {
+  return BrowserTestUtils.withNewTab(URL, async browser => {
+    return await SpecialPowers.spawn(browser, [], async function() {
+      const rect = content.document.documentElement.getBoundingClientRect();
+      info(
+        `${rect.width}x${rect.height}, ${content.innerWidth}x${content.innerHeight}`
+      );
+      is(
+        Math.round(rect.height),
+        content.innerHeight,
+        "Should fill the viewport and not overflow"
+      );
+    });
+  });
+}
+
+add_task(async function test_with_no_text_zoom() {
+  await test_once();
+});
+
+add_task(async function test_with_text_zoom() {
+  let dpi = window.devicePixelRatio;
+
+  await SpecialPowers.pushPrefEnv({ set: [["ui.textScaleFactor", 200]] });
+  ok(
+    window.devicePixelRatio > dpi,
+    "DPI should change as a result of the pref flip"
+  );
+
+  return test_once();
+});
diff --git a/layout/svg/SVGOuterSVGFrame.cpp b/layout/svg/SVGOuterSVGFrame.cpp
index 4d3e919197f8..7c2e57550af2 100644
--- a/layout/svg/SVGOuterSVGFrame.cpp
+++ b/layout/svg/SVGOuterSVGFrame.cpp
@@ -40,12 +40,7 @@ NS_IMPL_FRAMEARENA_HELPERS(SVGOuterSVGFrame)
 
 SVGOuterSVGFrame::SVGOuterSVGFrame(ComputedStyle* aStyle,
                                    nsPresContext* aPresContext)
-    : SVGDisplayContainerFrame(aStyle, aPresContext, kClassID),
-      mFullZoom(PresContext()->GetFullZoom()),
-      mCallingReflowSVG(false),
-      mIsRootContent(false),
-      mIsInObjectOrEmbed(false),
-      mIsInIframe(false) {
+    : SVGDisplayContainerFrame(aStyle, aPresContext, kClassID) {
   // Outer-<svg> has CSS layout, so remove this bit:
   RemoveStateBits(NS_FRAME_SVG_LAYOUT);
   AddStateBits(NS_FRAME_REFLOW_ROOT | NS_FRAME_FONT_INFLATION_CONTAINER |
@@ -66,6 +61,16 @@ static inline ContainSizeAxes ContainSizeAxesIfApplicable(
   return aFrame->GetContainSizeAxes();
 }
 
+// This should match ImageDocument::GetZoomLevel.
+float SVGOuterSVGFrame::ComputeFullZoom() const {
+  MOZ_ASSERT(mIsRootContent);
+  MOZ_ASSERT(!mIsInIframe);
+  if (BrowsingContext* bc = PresContext()->Document()->GetBrowsingContext()) {
+    return bc->FullZoom();
+  }
+  return 1.0f;
+}
+
 void SVGOuterSVGFrame::Init(nsIContent* aContent, nsContainerFrame* aParent,
                             nsIFrame* aPrevInFlow) {
   NS_ASSERTION(aContent->IsSVGElement(nsGkAtoms::svg),
@@ -98,6 +103,9 @@ void SVGOuterSVGFrame::Init(nsIContent* aContent, nsContainerFrame* aParent,
         mIsInIframe = nsGkAtoms::iframe->Equals(*type);
       }
     }
+    if (!mIsInIframe) {
+      mFullZoom = ComputeFullZoom();
+    }
   }
 
   MaybeSendIntrinsicSizeAndRatioToEmbedder();
@@ -255,7 +263,7 @@ nsIFrame::SizeComputationResult SVGOuterSVGFrame::ComputeSize(
   LogicalSize cbSize = aCBSize;
   IntrinsicSize intrinsicSize = GetIntrinsicSize();
 
-  if (!mContent->GetParent()) {
+  if (mIsRootContent) {
     // We're the root of the outermost browsing context, so we need to scale
     // cbSize by the full-zoom so that SVGs with percentage width/height zoom:
 
@@ -264,8 +272,11 @@ nsIFrame::SizeComputationResult SVGOuterSVGFrame::ComputeSize(
                  "root should not have auto-width/height containing block");
 
     if (!mIsInIframe) {
-      cbSize.ISize(aWritingMode) *= PresContext()->GetFullZoom();
-      cbSize.BSize(aWritingMode) *= PresContext()->GetFullZoom();
+      // NOTE: We can't just use mFullZoom because this can run before Reflow()
+      // updates it.
+      const float zoom = ComputeFullZoom();
+      cbSize.ISize(aWritingMode) *= zoom;
+      cbSize.BSize(aWritingMode) *= zoom;
     }
 
     // We also need to honour the width and height attributes' default values
@@ -274,7 +285,7 @@ nsIFrame::SizeComputationResult SVGOuterSVGFrame::ComputeSize(
     // intrinsic size.  Also note that explicit percentage values are mapped
     // into style, so the following isn't for them.)
 
-    SVGSVGElement* content = static_cast<SVGSVGElement*>(GetContent());
+    auto* content = static_cast<SVGSVGElement*>(GetContent());
 
     const SVGAnimatedLength& width =
         content->mLengthAttributes[SVGSVGElement::ATTR_WIDTH];
@@ -383,9 +394,12 @@ void SVGOuterSVGFrame::Reflow(nsPresContext* aPresContext,
     changeBits |= COORD_CONTEXT_CHANGED;
     svgElem->SetViewportSize(newViewportSize);
   }
-  if (mFullZoom != PresContext()->GetFullZoom() && !mIsInIframe) {
-    changeBits |= FULL_ZOOM_CHANGED;
-    mFullZoom = PresContext()->GetFullZoom();
+  if (mIsRootContent && !mIsInIframe) {
+    const auto oldZoom = mFullZoom;
+    mFullZoom = ComputeFullZoom();
+    if (oldZoom != mFullZoom) {
+      changeBits |= FULL_ZOOM_CHANGED;
+    }
   }
   if (changeBits && !HasAnyStateBits(NS_FRAME_FIRST_REFLOW)) {
     NotifyViewportOrTransformChanged(changeBits);
diff --git a/layout/svg/SVGOuterSVGFrame.h b/layout/svg/SVGOuterSVGFrame.h
index 8db256c690a4..68ac7caba585 100644
--- a/layout/svg/SVGOuterSVGFrame.h
+++ b/layout/svg/SVGOuterSVGFrame.h
@@ -143,17 +143,18 @@ class SVGOuterSVGFrame final : public SVGDisplayContainerFrame,
    * being used as an image.
    */
   bool IsRootOfImage();
+  float ComputeFullZoom() const;
 
   void MaybeSendIntrinsicSizeAndRatioToEmbedder();
   void MaybeSendIntrinsicSizeAndRatioToEmbedder(Maybe<IntrinsicSize>,
                                                 Maybe<AspectRatio>);
 
-  float mFullZoom;
+  float mFullZoom = 1.0f;
 
-  bool mCallingReflowSVG;
-  bool mIsRootContent;
-  bool mIsInObjectOrEmbed;
-  bool mIsInIframe;
+  bool mCallingReflowSVG = false;
+  bool mIsRootContent = false;
+  bool mIsInObjectOrEmbed = false;
+  bool mIsInIframe = false;
 };
 
 ////////////////////////////////////////////////////////////////////////