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diff --git a/dom/base/nsViewportInfo.h b/dom/base/nsViewportInfo.h
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+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef nsViewportInfo_h___
+#define nsViewportInfo_h___
+
+#include <algorithm>
+#include <stdint.h>
+#include "mozilla/Attributes.h"
+#include "mozilla/StaticPrefs_apz.h"
+#include "Units.h"
+
+namespace mozilla::dom {
+enum class ViewportFitType : uint8_t {
+  Auto,
+  Contain,
+  Cover,
+};
+}  // namespace mozilla::dom
+
+/**
+ * Default values for the nsViewportInfo class.
+ */
+static const mozilla::CSSIntSize kViewportMinSize(200, 40);
+static const mozilla::CSSIntSize kViewportMaxSize(10000, 10000);
+
+inline mozilla::LayoutDeviceToScreenScale ViewportMinScale() {
+  return mozilla::LayoutDeviceToScreenScale(
+      std::max(mozilla::StaticPrefs::apz_min_zoom(), 0.1f));
+}
+
+inline mozilla::LayoutDeviceToScreenScale ViewportMaxScale() {
+  return mozilla::LayoutDeviceToScreenScale(
+      std::min(mozilla::StaticPrefs::apz_max_zoom(), 100.0f));
+}
+
+/**
+ * Information retrieved from the <meta name="viewport"> tag. See
+ * Document::GetViewportInfo for more information on this functionality.
+ */
+class MOZ_STACK_CLASS nsViewportInfo {
+ public:
+  enum class AutoSizeFlag {
+    AutoSize,
+    FixedSize,
+  };
+  enum class AutoScaleFlag {
+    AutoScale,
+    FixedScale,
+  };
+  enum class ZoomFlag {
+    AllowZoom,
+    DisallowZoom,
+  };
+  enum class ZoomBehaviour {
+    Mobile,
+    Desktop,  // disallows zooming out past default zoom
+  };
+  nsViewportInfo(const mozilla::ScreenIntSize& aDisplaySize,
+                 const mozilla::CSSToScreenScale& aDefaultZoom,
+                 ZoomFlag aZoomFlag, ZoomBehaviour aBehaviour,
+                 AutoScaleFlag aAutoScaleFlag = AutoScaleFlag::FixedScale)
+      : mDefaultZoom(aDefaultZoom),
+        mViewportFit(mozilla::dom::ViewportFitType::Auto),
+        mDefaultZoomValid(aAutoScaleFlag != AutoScaleFlag::AutoScale),
+        mAutoSize(true),
+        mAllowZoom(aZoomFlag == ZoomFlag::AllowZoom) {
+    mSize = mozilla::ScreenSize(aDisplaySize) / mDefaultZoom;
+    mozilla::CSSToLayoutDeviceScale pixelRatio(1.0f);
+    if (aBehaviour == ZoomBehaviour::Desktop) {
+      mMinZoom = aDefaultZoom;
+    } else {
+      mMinZoom = pixelRatio * ViewportMinScale();
+    }
+    mMaxZoom = pixelRatio * ViewportMaxScale();
+    ConstrainViewportValues();
+  }
+
+  nsViewportInfo(const mozilla::CSSToScreenScale& aDefaultZoom,
+                 const mozilla::CSSToScreenScale& aMinZoom,
+                 const mozilla::CSSToScreenScale& aMaxZoom,
+                 const mozilla::CSSSize& aSize, AutoSizeFlag aAutoSizeFlag,
+                 AutoScaleFlag aAutoScaleFlag, ZoomFlag aZoomFlag,
+                 mozilla::dom::ViewportFitType aViewportFit)
+      : mDefaultZoom(aDefaultZoom),
+        mMinZoom(aMinZoom),
+        mMaxZoom(aMaxZoom),
+        mSize(aSize),
+        mViewportFit(aViewportFit),
+        mDefaultZoomValid(aAutoScaleFlag != AutoScaleFlag::AutoScale),
+        mAutoSize(aAutoSizeFlag == AutoSizeFlag::AutoSize),
+        mAllowZoom(aZoomFlag == ZoomFlag::AllowZoom) {
+    ConstrainViewportValues();
+  }
+
+  bool IsDefaultZoomValid() const { return mDefaultZoomValid; }
+  mozilla::CSSToScreenScale GetDefaultZoom() const { return mDefaultZoom; }
+  mozilla::CSSToScreenScale GetMinZoom() const { return mMinZoom; }
+  mozilla::CSSToScreenScale GetMaxZoom() const { return mMaxZoom; }
+
+  mozilla::CSSSize GetSize() const { return mSize; }
+
+  bool IsAutoSizeEnabled() const { return mAutoSize; }
+  bool IsZoomAllowed() const { return mAllowZoom; }
+
+  mozilla::dom::ViewportFitType GetViewportFit() const { return mViewportFit; }
+
+  static constexpr float kAuto = -1.0f;
+  static constexpr float kExtendToZoom = -2.0f;
+  static constexpr float kDeviceSize =
+      -3.0f;  // for device-width or device-height
+
+  // MIN/MAX computations where one of the arguments is auto resolve to the
+  // other argument. For instance, MIN(0.25, auto) = 0.25, and
+  // MAX(5, auto) = 5.
+  // https://drafts.csswg.org/css-device-adapt/#constraining-defs
+  static const float& Max(const float& aA, const float& aB);
+  static const float& Min(const float& aA, const float& aB);
+
+ private:
+  /**
+   * Constrain the viewport calculations from the
+   * Document::GetViewportInfo() function in order to always return
+   * sane minimum/maximum values.
+   */
+  void ConstrainViewportValues();
+
+  // Default zoom indicates the level at which the display is 'zoomed in'
+  // initially for the user, upon loading of the page.
+  mozilla::CSSToScreenScale mDefaultZoom;
+
+  // The minimum zoom level permitted by the page.
+  mozilla::CSSToScreenScale mMinZoom;
+
+  // The maximum zoom level permitted by the page.
+  mozilla::CSSToScreenScale mMaxZoom;
+
+  // The size of the viewport, specified by the <meta name="viewport"> tag.
+  mozilla::CSSSize mSize;
+
+  // The value of the viewport-fit.
+  mozilla::dom::ViewportFitType mViewportFit;
+
+  // If the default zoom was specified and was between the min and max
+  // zoom values.
+  // FIXME: Bug 1504362 - Unify this and mDefaultZoom into
+  // Maybe<CSSToScreenScale>.
+  bool mDefaultZoomValid;
+
+  // Whether or not we should automatically size the viewport to the device's
+  // width. This is true if the document has been optimized for mobile, and
+  // the width property of a specified <meta name="viewport"> tag is either
+  // not specified, or is set to the special value 'device-width'.
+  bool mAutoSize;
+
+  // Whether or not the user can zoom in and out on the page. Default is true.
+  bool mAllowZoom;
+};
+
+#endif