Bug 1687183 - Fix dark background detection in nsNativeTheme when only root has a background. r=mstange

If we hit the canvas frame, and the root has a background but the body
doesn't, previously we were using the body style. Then
FindNonTransparentBackgroundFrame would return the document element
frame (with the right background), but FindBackgroundFor that failed.

Differential Revision: https://phabricator.services.mozilla.com/D103975

layout/style/nsStyleStruct.cpp

@@ -2065,7 +2065,7 @@ nscolor nsStyleBackground::BackgroundColor(const nsIFrame* aFrame) const {
   return mBackgroundColor.CalcColor(aFrame);
 }
 
-nscolor nsStyleBackground::BackgroundColor(ComputedStyle* aStyle) const {
+nscolor nsStyleBackground::BackgroundColor(const ComputedStyle* aStyle) const {
   return mBackgroundColor.CalcColor(*aStyle);
 }
 

layout/style/nsStyleStruct.h

@@ -342,7 +342,7 @@ struct MOZ_NEEDS_MEMMOVABLE_MEMBERS nsStyleBackground {
 
   // Return the background color as nscolor.
   nscolor BackgroundColor(const nsIFrame* aFrame) const;
-  nscolor BackgroundColor(mozilla::ComputedStyle* aStyle) const;
+  nscolor BackgroundColor(const mozilla::ComputedStyle* aStyle) const;
 
   // True if this background is completely transparent.
   bool IsTransparent(const nsIFrame* aFrame) const;

widget/nsNativeTheme.cpp

@@ -607,17 +607,39 @@ bool nsNativeTheme::IsRangeHorizontal(nsIFrame* aFrame) {
 }
 
 static nsIFrame* GetBodyFrame(nsIFrame* aCanvasFrame) {
-  nsIContent* content = aCanvasFrame->GetContent();
-  if (!content) {
-    return nullptr;
-  }
-  nsIContent* body = content->OwnerDoc()->GetBodyElement();
+  nsIContent* body = aCanvasFrame->PresContext()->Document()->GetBodyElement();
   if (!body) {
     return nullptr;
   }
   return body->GetPrimaryFrame();
 }
 
+static const ComputedStyle* GetBackgroundStyle(nsIFrame* aFrame) {
+  if (nsCSSRendering::IsCanvasFrame(aFrame)) {
+    // For canvas frames, prefer to look at the body first, because the body
+    // background color is most likely what will be visible as the background
+    // color of the page, even if the html element has a different background
+    // color which prevents that of the body frame to propagate to the viewport.
+    if (nsIFrame* bodyFrame = GetBodyFrame(aFrame)) {
+      if (!bodyFrame->StyleBackground()->IsTransparent(bodyFrame->Style())) {
+        return bodyFrame->Style();
+      }
+    }
+  }
+  ComputedStyle* bgSC = nullptr;
+  if (nsCSSRendering::FindBackground(aFrame, &bgSC) &&
+      !bgSC->StyleBackground()->IsTransparent(bgSC)) {
+    return bgSC;
+  }
+
+  nsIFrame* backgroundFrame =
+      nsCSSRendering::FindNonTransparentBackgroundFrame(aFrame, true);
+  if (!backgroundFrame) {
+    return nullptr;
+  }
+  return backgroundFrame->Style();
+}
+
 /* static */
 bool nsNativeTheme::IsDarkBackground(nsIFrame* aFrame) {
   nsIScrollableFrame* scrollFrame = nullptr;
@@ -625,28 +647,12 @@ bool nsNativeTheme::IsDarkBackground(nsIFrame* aFrame) {
     scrollFrame = aFrame->GetScrollTargetFrame();
     aFrame = aFrame->GetParent();
   }
-  if (!scrollFrame) return false;
+  if (!scrollFrame) {
+    return false;
+  }
 
-  nsIFrame* frame = scrollFrame->GetScrolledFrame();
-  if (nsCSSRendering::IsCanvasFrame(frame)) {
-    // For canvas frames, prefer to look at the body first, because the body
-    // background color is most likely what will be visible as the background
-    // color of the page, even if the html element has a different background
-    // color which prevents that of the body frame to propagate to the viewport.
-    nsIFrame* bodyFrame = GetBodyFrame(frame);
-    if (bodyFrame) {
-      frame = bodyFrame;
-    }
-  }
-  ComputedStyle* bgSC = nullptr;
-  if (!nsCSSRendering::FindBackground(frame, &bgSC) ||
-      bgSC->StyleBackground()->IsTransparent(bgSC)) {
-    nsIFrame* backgroundFrame =
-        nsCSSRendering::FindNonTransparentBackgroundFrame(frame, true);
-    nsCSSRendering::FindBackground(backgroundFrame, &bgSC);
-  }
-  if (bgSC) {
-    nscolor bgColor = bgSC->StyleBackground()->BackgroundColor(bgSC);
+  if (const auto* style = GetBackgroundStyle(scrollFrame->GetScrolledFrame())) {
+    nscolor bgColor = style->StyleBackground()->BackgroundColor(style);
     // Consider the background color dark if the sum of the r, g and b values is
     // less than 384 in a semi-transparent document.  This heuristic matches
     // what WebKit does, and we can improve it later if needed.