Bug 1744749 - Add an API to get the real canvas background color. r=jwatt

Extensions are using getComputedStyle(body).backgroundColor, which is
wrong at multiple levels.

The one that matters for this bug is that it is not color-scheme aware.

Depends on D133770

Differential Revision: https://phabricator.services.mozilla.com/D133771

dom/base/nsDOMWindowUtils.cpp

@@ -2115,6 +2115,19 @@ nsDOMWindowUtils::GetNodeObservedByIMEContentObserver(nsINode** aNode) {
   return NS_OK;
 }
 
+NS_IMETHODIMP
+nsDOMWindowUtils::GetCanvasBackgroundColor(nsAString& aColor) {
+  if (RefPtr<Document> doc = GetDocument()) {
+    doc->FlushPendingNotifications(FlushType::Frames);
+  }
+  nscolor color = NS_RGB(255, 255, 255);
+  if (PresShell* presShell = GetPresShell()) {
+    color = presShell->ComputeCanvasBackground().mColor;
+  }
+  nsStyleUtil::GetSerializedColorValue(color, aColor);
+  return NS_OK;
+}
+
 NS_IMETHODIMP
 nsDOMWindowUtils::GetFocusedInputType(nsAString& aType) {
   nsCOMPtr<nsIWidget> widget = GetWidget();

dom/interfaces/base/nsIDOMWindowUtils.idl

@@ -1687,6 +1687,11 @@ interface nsIDOMWindowUtils : nsISupports {
                                      in AString aProperty,
                                      in long aFlushType);
 
+  /**
+   * Returns the effective canvas background color for the window.
+   */
+  readonly attribute AString canvasBackgroundColor;
+
   /**
    * Get the type of the currently focused html input, if any.
    */

layout/base/PresShell.cpp

@@ -5325,7 +5325,7 @@ static bool IsTransparentContainerElement(nsPresContext* aPresContext) {
   return false;
 }
 
-nscolor PresShell::GetDefaultBackgroundColorToDraw() {
+nscolor PresShell::GetDefaultBackgroundColorToDraw() const {
   if (!mPresContext || !mPresContext->GetBackgroundColorDraw()) {
     return NS_RGB(255, 255, 255);
   }
@@ -5356,47 +5356,51 @@ nscolor PresShell::GetDefaultBackgroundColorToDraw() {
 }
 
 void PresShell::UpdateCanvasBackground() {
+  auto canvasBg = ComputeCanvasBackground();
+  mCanvasBackgroundColor = canvasBg.mColor;
+  mHasCSSBackgroundColor = canvasBg.mCSSSpecified;
+}
+
+PresShell::CanvasBackground PresShell::ComputeCanvasBackground() const {
   // If we have a frame tree and it has style information that
   // specifies the background color of the canvas, update our local
   // cache of that color.
   nsIFrame* rootStyleFrame = FrameConstructor()->GetRootElementStyleFrame();
-  if (rootStyleFrame) {
-    ComputedStyle* bgStyle =
-        nsCSSRendering::FindRootFrameBackground(rootStyleFrame);
-    // XXX We should really be passing the canvasframe, not the root element
-    // style frame but we don't have access to the canvasframe here. It isn't
-    // a problem because only a few frames can return something other than true
-    // and none of them would be a canvas frame or root element style frame.
-    bool drawBackgroundImage = false;
-    bool drawBackgroundColor = false;
-    const nsStyleDisplay* disp = rootStyleFrame->StyleDisplay();
-    StyleAppearance appearance = disp->EffectiveAppearance();
-    if (rootStyleFrame->IsThemed(disp) &&
-        appearance != StyleAppearance::MozWinGlass &&
-        appearance != StyleAppearance::MozWinBorderlessGlass) {
-      // Ignore the CSS background-color if -moz-appearance is used and it is
-      // not one of the glass values. (Windows 7 Glass has traditionally not
-      // overridden background colors, so we preserve that behavior for now.)
-      mCanvasBackgroundColor = NS_RGBA(0, 0, 0, 0);
-    } else {
-      mCanvasBackgroundColor = nsCSSRendering::DetermineBackgroundColor(
-          mPresContext, bgStyle, rootStyleFrame, drawBackgroundImage,
-          drawBackgroundColor);
-    }
-    mHasCSSBackgroundColor = drawBackgroundColor;
-    if (mPresContext->IsRootContentDocumentCrossProcess() &&
-        !IsTransparentContainerElement(mPresContext)) {
-      mCanvasBackgroundColor = NS_ComposeColors(
-          GetDefaultBackgroundColorToDraw(), mCanvasBackgroundColor);
-    }
+  if (!rootStyleFrame) {
+    // If the root element of the document (ie html) has style 'display: none'
+    // then the document's background color does not get drawn; return the color
+    // we actually draw.
+    return {GetDefaultBackgroundColorToDraw(), false};
   }
 
-  // If the root element of the document (ie html) has style 'display: none'
-  // then the document's background color does not get drawn; cache the
-  // color we actually draw.
-  if (!FrameConstructor()->GetRootElementFrame()) {
-    mCanvasBackgroundColor = GetDefaultBackgroundColorToDraw();
+  ComputedStyle* bgStyle =
+      nsCSSRendering::FindRootFrameBackground(rootStyleFrame);
+  // XXX We should really be passing the canvasframe, not the root element
+  // style frame but we don't have access to the canvasframe here. It isn't
+  // a problem because only a few frames can return something other than true
+  // and none of them would be a canvas frame or root element style frame.
+  nscolor color = NS_RGBA(0, 0, 0, 0);
+  bool drawBackgroundImage = false;
+  bool drawBackgroundColor = false;
+  const nsStyleDisplay* disp = rootStyleFrame->StyleDisplay();
+  StyleAppearance appearance = disp->EffectiveAppearance();
+  if (rootStyleFrame->IsThemed(disp) &&
+      appearance != StyleAppearance::MozWinGlass &&
+      appearance != StyleAppearance::MozWinBorderlessGlass) {
+    // Ignore the CSS background-color if -moz-appearance is used and it is
+    // not one of the glass values. (Windows 7 Glass has traditionally not
+    // overridden background colors, so we preserve that behavior for now.)
+  } else {
+    color = nsCSSRendering::DetermineBackgroundColor(
+        mPresContext, bgStyle, rootStyleFrame, drawBackgroundImage,
+        drawBackgroundColor);
   }
+  if (mPresContext->IsRootContentDocumentCrossProcess() &&
+      !IsTransparentContainerElement(mPresContext)) {
+    color = NS_ComposeColors(
+        GetDefaultBackgroundColorToDraw(), color);
+  }
+  return {color, drawBackgroundColor};
 }
 
 nscolor PresShell::ComputeBackstopColor(nsView* aDisplayRoot) {

layout/base/PresShell.h

@@ -877,12 +877,17 @@ class PresShell final : public nsStubDocumentObserver,
    * bug 488242, bug 476557 and other bugs mentioned there.
    */
   void SetCanvasBackground(nscolor aColor) { mCanvasBackgroundColor = aColor; }
-  nscolor GetCanvasBackground() { return mCanvasBackgroundColor; }
+  nscolor GetCanvasBackground() const { return mCanvasBackgroundColor; }
 
   /**
-   * Use the current frame tree (if it exists) to update the background
-   * color of the most recently drawn canvas.
+   * Use the current frame tree (if it exists) to update the background color of
+   * the most recently drawn canvas.
    */
+  struct CanvasBackground {
+    nscolor mColor = 0;
+    bool mCSSSpecified = false;
+  };
+  CanvasBackground ComputeCanvasBackground() const;
   void UpdateCanvasBackground();
 
   /**
@@ -2760,7 +2765,7 @@ class PresShell final : public nsStubDocumentObserver,
 
   PresShell* GetRootPresShell() const;
 
-  nscolor GetDefaultBackgroundColorToDraw();
+  nscolor GetDefaultBackgroundColorToDraw() const;
 
   //////////////////////////////////////////////////////////////////////////////
   // Approximate frame visibility tracking implementation.