From a8a5c8fe6a2b8692f9b9d12f50b40155abfa8819 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Emilio=20Cobos=20=C3=81lvarez?= <emilio@crisal.io>
Date: Wed, 16 Nov 2022 15:52:07 +0000
Subject: [PATCH] Bug 1798213 - For widget-less pages keep defaulting to
 primary screen scale factor.
 r=tnikkel,layout-reviewers,extension-reviewers,robwu

This restores the previous behavior in a somewhat more principled way.

The extensions code is still broken in multi-monitor cases, but that's a
more complicated fix.

Differential Revision: https://phabricator.services.mozilla.com/D161997
---
 dom/ipc/BrowserParent.cpp                     | 49 ++++++++++---------
 .../test/mochitest/mochitest-common.ini       |  1 +
 .../test_ext_background_page_dpi.html         | 46 +++++++++++++++++
 widget/PuppetWidget.h                         |  4 +-
 widget/Screen.cpp                             | 20 +++++---
 widget/Screen.h                               |  3 ++
 widget/nsBaseWidget.cpp                       | 12 +++++
 widget/nsIWidget.h                            | 10 ++++
 8 files changed, 114 insertions(+), 31 deletions(-)
 create mode 100644 toolkit/components/extensions/test/mochitest/test_ext_background_page_dpi.html

diff --git a/dom/ipc/BrowserParent.cpp b/dom/ipc/BrowserParent.cpp
index 567a8abfec9f..d16a45f42a98 100644
--- a/dom/ipc/BrowserParent.cpp
+++ b/dom/ipc/BrowserParent.cpp
@@ -473,10 +473,10 @@ ParentShowInfo BrowserParent::GetShowInfo() {
   TryCacheDPIAndScale();
   if (mFrameElement) {
     nsAutoString name;
-    mFrameElement->GetAttr(kNameSpaceID_None, nsGkAtoms::name, name);
+    mFrameElement->GetAttr(nsGkAtoms::name, name);
     bool isTransparent =
         nsContentUtils::IsChromeDoc(mFrameElement->OwnerDoc()) &&
-        mFrameElement->HasAttr(kNameSpaceID_None, nsGkAtoms::transparent);
+        mFrameElement->HasAttr(nsGkAtoms::transparent);
     return ParentShowInfo(name, false, isTransparent, mDPI, mRounding,
                           mDefaultScale.scale);
   }
@@ -546,6 +546,10 @@ void BrowserParent::SetOwnerElement(Element* aElement) {
 #endif
 
   AddWindowListeners();
+
+  // The DPI depends on our frame element's widget, so invalidate now in case
+  // we've tried to cache it already.
+  mDPI = -1;
   TryCacheDPIAndScale();
 
   if (mRemoteLayerTreeOwner.IsInitialized()) {
@@ -3427,17 +3431,17 @@ void BrowserParent::TryCacheDPIAndScale() {
     return;
   }
 
+  const auto oldDefaultScale = mDefaultScale;
   nsCOMPtr<nsIWidget> widget = GetWidget();
+  mDPI = widget ? widget->GetDPI() : nsIWidget::GetFallbackDPI();
+  mRounding = widget ? widget->RoundsWidgetCoordinatesTo() : 1;
+  mDefaultScale =
+      widget ? widget->GetDefaultScale() : nsIWidget::GetFallbackDefaultScale();
 
-  if (widget) {
-    mDPI = widget->GetDPI();
-    mRounding = widget->RoundsWidgetCoordinatesTo();
-    if (mDefaultScale != widget->GetDefaultScale()) {
-      // The change of the default scale factor will affect the child dimensions
-      // so we need to invalidate it.
-      mUpdatedDimensions = false;
-    }
-    mDefaultScale = widget->GetDefaultScale();
+  if (mDefaultScale != oldDefaultScale) {
+    // The change of the default scale factor will affect the child dimensions
+    // so we need to invalidate it.
+    mUpdatedDimensions = false;
   }
 }
 
@@ -3582,18 +3586,19 @@ void BrowserParent::PreserveLayers(bool aPreserveLayers) {
 }
 
 void BrowserParent::NotifyResolutionChanged() {
-  if (!mIsDestroyed) {
-    // TryCacheDPIAndScale()'s cache is keyed off of
-    // mDPI being greater than 0, so this invalidates it.
-    mDPI = -1;
-    TryCacheDPIAndScale();
-    // If mDPI was set to -1 to invalidate it and then TryCacheDPIAndScale
-    // fails to cache the values, then mDefaultScale.scale might be invalid.
-    // We don't want to send that value to content. Just send -1 for it too in
-    // that case.
-    Unused << SendUIResolutionChanged(mDPI, mRounding,
-                                      mDPI < 0 ? -1.0 : mDefaultScale.scale);
+  if (mIsDestroyed) {
+    return;
   }
+  // TryCacheDPIAndScale()'s cache is keyed off of
+  // mDPI being greater than 0, so this invalidates it.
+  mDPI = -1;
+  TryCacheDPIAndScale();
+  // If mDPI was set to -1 to invalidate it and then TryCacheDPIAndScale
+  // fails to cache the values, then mDefaultScale.scale might be invalid.
+  // We don't want to send that value to content. Just send -1 for it too in
+  // that case.
+  Unused << SendUIResolutionChanged(mDPI, mRounding,
+                                    mDPI < 0 ? -1.0 : mDefaultScale.scale);
 }
 
 bool BrowserParent::CanCancelContentJS(
diff --git a/toolkit/components/extensions/test/mochitest/mochitest-common.ini b/toolkit/components/extensions/test/mochitest/mochitest-common.ini
index 805d79b0b558..77a8631bcb0c 100644
--- a/toolkit/components/extensions/test/mochitest/mochitest-common.ini
+++ b/toolkit/components/extensions/test/mochitest/mochitest-common.ini
@@ -87,6 +87,7 @@ skip-if = toolkit == 'android' || tsan # near-permafail after landing bug 127005
 [test_ext_background_canvas.html]
 [test_ext_background_page.html]
 skip-if = (toolkit == 'android') # android doesn't have devtools
+[test_ext_background_page_dpi.html]
 [test_ext_browserAction_openPopup.html]
 [test_ext_browserAction_openPopup_incognito_window.html]
 skip-if = os == "android" # cannot open private windows - bug 1372178
diff --git a/toolkit/components/extensions/test/mochitest/test_ext_background_page_dpi.html b/toolkit/components/extensions/test/mochitest/test_ext_background_page_dpi.html
new file mode 100644
index 000000000000..40772402b143
--- /dev/null
+++ b/toolkit/components/extensions/test/mochitest/test_ext_background_page_dpi.html
@@ -0,0 +1,46 @@
+<!DOCTYPE HTML>
+<meta charset="utf-8">
+<title>DPI of background page</title>
+<script src="/tests/SimpleTest/SimpleTest.js"></script>
+<script src="/tests/SimpleTest/ExtensionTestUtils.js"></script>
+<script src="head.js"></script>
+<link rel="stylesheet" href="/tests/SimpleTest/test.css">
+<script>
+"use strict";
+
+async function testDPIMatches(description) {
+  let extension = ExtensionTestUtils.loadExtension({
+    background: function() {
+      browser.test.sendMessage("dpi", window.devicePixelRatio);
+    },
+  });
+  await extension.startup();
+  let dpi = await extension.awaitMessage("dpi");
+  await extension.unload();
+
+  // This assumes that the window is loaded in a device DPI.
+  is(
+    dpi,
+    window.devicePixelRatio,
+    `DPI in a background page should match DPI in primary chrome page ${description}`
+  );
+}
+
+add_task(async function test_dpi_simple() {
+  await testDPIMatches("by default");
+});
+
+add_task(async function test_dpi_devPixelsPerPx() {
+  await SpecialPowers.pushPrefEnv({
+    set: [["layout.css.devPixelsPerPx", 1.5]],
+  });
+  await testDPIMatches("with devPixelsPerPx");
+  await SpecialPowers.popPrefEnv();
+});
+
+add_task(async function test_dpi_os_zoom() {
+  await SpecialPowers.pushPrefEnv({ set: [["ui.textScaleFactor", 200]] });
+  await testDPIMatches("with OS zoom");
+  await SpecialPowers.popPrefEnv();
+});
+</script>
diff --git a/widget/PuppetWidget.h b/widget/PuppetWidget.h
index d78c35950c12..27b32c2bde11 100644
--- a/widget/PuppetWidget.h
+++ b/widget/PuppetWidget.h
@@ -382,9 +382,9 @@ class PuppetWidget : public nsBaseWidget,
   ContentCacheInChild mContentCache;
 
   // The DPI of the parent widget containing this widget.
-  float mDPI = 96;
+  float mDPI = GetFallbackDPI();
   int32_t mRounding = 1;
-  double mDefaultScale = 1.0f;
+  double mDefaultScale = GetFallbackDefaultScale().scale;
 
   ScreenIntMargin mSafeAreaInsets;
 
diff --git a/widget/Screen.cpp b/widget/Screen.cpp
index 4962d1c9bac7..19a95acba522 100644
--- a/widget/Screen.cpp
+++ b/widget/Screen.cpp
@@ -129,15 +129,21 @@ Screen::GetContentsScaleFactor(double* aOutScale) {
   return NS_OK;
 }
 
+CSSToLayoutDeviceScale Screen::GetCSSToLayoutDeviceScale(
+    IncludeOSZoom aIncludeOSZoom) const {
+  auto scale = CSSToLayoutDeviceScale(StaticPrefs::layout_css_devPixelsPerPx());
+  if (scale.scale <= 0.0) {
+    scale = mDefaultCssScale;
+  }
+  if (bool(aIncludeOSZoom)) {
+    scale.scale *= LookAndFeel::SystemZoomSettings().mFullZoom;
+  }
+  return scale;
+}
+
 NS_IMETHODIMP
 Screen::GetDefaultCSSScaleFactor(double* aOutScale) {
-  double scale = StaticPrefs::layout_css_devPixelsPerPx();
-  if (scale > 0.0) {
-    *aOutScale = scale;
-  } else {
-    *aOutScale = mDefaultCssScale.scale;
-  }
-  *aOutScale *= LookAndFeel::SystemZoomSettings().mFullZoom;
+  *aOutScale = GetCSSToLayoutDeviceScale(IncludeOSZoom::Yes).scale;
   return NS_OK;
 }
 
diff --git a/widget/Screen.h b/widget/Screen.h
index ac890e8ab26f..056a45a51d7f 100644
--- a/widget/Screen.h
+++ b/widget/Screen.h
@@ -51,6 +51,9 @@ class Screen final : public nsIScreen {
     return mContentsScale;
   }
 
+  enum class IncludeOSZoom : bool { No, Yes };
+  CSSToLayoutDeviceScale GetCSSToLayoutDeviceScale(IncludeOSZoom) const;
+
  private:
   virtual ~Screen() = default;
 
diff --git a/widget/nsBaseWidget.cpp b/widget/nsBaseWidget.cpp
index f3b47d625f29..c5756a2fee7b 100644
--- a/widget/nsBaseWidget.cpp
+++ b/widget/nsBaseWidget.cpp
@@ -59,6 +59,7 @@
 #include "mozilla/layers/InputAPZContext.h"
 #include "mozilla/layers/WebRenderLayerManager.h"
 #include "mozilla/webrender/WebRenderTypes.h"
+#include "mozilla/widget/ScreenManager.h"
 #include "nsAppDirectoryServiceDefs.h"
 #include "nsCOMPtr.h"
 #include "nsContentUtils.h"
@@ -2105,6 +2106,17 @@ void nsIWidget::OnLongTapTimerCallback(nsITimer* aTimer, void* aClosure) {
   self->mLongTapTouchPoint = nullptr;
 }
 
+float nsIWidget::GetFallbackDPI() {
+  RefPtr<const Screen> primaryScreen =
+      ScreenManager::GetSingleton().GetPrimaryScreen();
+  return primaryScreen->GetDPI();
+}
+
+CSSToLayoutDeviceScale nsIWidget::GetFallbackDefaultScale() {
+  RefPtr<const Screen> s = ScreenManager::GetSingleton().GetPrimaryScreen();
+  return s->GetCSSToLayoutDeviceScale(Screen::IncludeOSZoom::No);
+}
+
 nsresult nsIWidget::ClearNativeTouchSequence(nsIObserver* aObserver) {
   AutoObserverNotifier notifier(aObserver, "cleartouch");
 
diff --git a/widget/nsIWidget.h b/widget/nsIWidget.h
index 3c5e39a9c2d7..b8db3c0973d6 100644
--- a/widget/nsIWidget.h
+++ b/widget/nsIWidget.h
@@ -594,6 +594,11 @@ class nsIWidget : public nsISupports {
    */
   virtual float GetDPI() = 0;
 
+  /**
+   * Fallback DPI for when there's no widget available.
+   */
+  static float GetFallbackDPI();
+
   /**
    * Return the scaling factor between device pixels and the platform-
    * dependent "desktop pixels" used to manage window positions on a
@@ -618,6 +623,11 @@ class nsIWidget : public nsISupports {
    */
   mozilla::CSSToLayoutDeviceScale GetDefaultScale();
 
+  /**
+   * Fallback default scale for when there's no widget available.
+   */
+  static mozilla::CSSToLayoutDeviceScale GetFallbackDefaultScale();
+
   /**
    * Return the first child of this widget.  Will return null if
    * there are no children.