Bug 1578377 - Render dark scrollbars for element with dark background on Windows. r=jmathies

Differential Revision: https://phabricator.services.mozilla.com/D48289

--HG--
extra : moz-landing-system : lando

layout/tools/reftest/runreftest.py

@@ -362,6 +362,8 @@ class RefTest(object):
         prefs['reftest.logLevel'] = options.log_tbpl_level or 'info'
         prefs['reftest.suite'] = options.suite
         prefs['gfx.font_rendering.ahem_antialias_none'] = True
+        # Disable dark scrollbars because it's semi-transparent.
+        prefs['widget.disable-dark-scrollbar'] = True
 
         # Set tests to run or manifests to parse.
         if tests:

modules/libpref/init/StaticPrefList.yaml

@@ -7496,6 +7496,16 @@
   value: false
   mirror: always
 
+# Preference to disable dark scrollbar implementation.
+# This is mainly for testing because dark scrollbars have to be semi-
+# transparent, but many reftests expect scrollbars to look identical
+# among different backgrounds.
+# However, some users may want to disable this as well.
+- name: widget.disable-dark-scrollbar
+  type: bool
+  value: false
+  mirror: always
+
 - name: widget.window-transforms.disabled
   type: RelaxedAtomicBool
   value: false

testing/profiles/web-platform/user.js

@@ -40,6 +40,9 @@ user_pref("dom.animations-api.implicit-keyframes.enabled", true);
 // sometime wpt runs test even before the document becomes visible, which would
 // delay video.play() and cause play() running in wrong order.
 user_pref("media.block-autoplay-until-in-foreground", false);
+// Disable dark scrollbars as it can be semi-transparent that many reftests
+// don't expect.
+user_pref("widget.disable-dark-scrollbar", true);
 user_pref("media.block-autoplay-until-in-foreground", false);
 // Enable AppCache globally for now whilst it's being removed in Bug 1584984
 user_pref("browser.cache.offline.storage.enable", true);

widget/nsNativeTheme.cpp

@@ -652,6 +652,7 @@ static nsIFrame* GetBodyFrame(nsIFrame* aCanvasFrame) {
   return body->GetPrimaryFrame();
 }
 
+/* static */
 bool nsNativeTheme::IsDarkBackground(nsIFrame* aFrame) {
   nsIScrollableFrame* scrollFrame = nullptr;
   while (!scrollFrame && aFrame) {

widget/nsNativeTheme.h

@@ -185,7 +185,7 @@ class nsNativeTheme : public nsITimerCallback, public nsINamed {
   bool IsRangeHorizontal(nsIFrame* aFrame);
 
   // scrollbar
-  bool IsDarkBackground(nsIFrame* aFrame);
+  static bool IsDarkBackground(nsIFrame* aFrame);
   // custom scrollbar
   typedef nscolor (*AutoColorGetter)(mozilla::ComputedStyle*);
   bool IsWidgetScrollbarPart(mozilla::StyleAppearance aAppearance);

widget/windows/nsNativeThemeWin.cpp

@@ -1514,9 +1514,26 @@ static bool IsScrollbarWidthThin(nsIFrame* aFrame) {
   return IsScrollbarWidthThin(style);
 }
 
-static bool ShouldDrawCustomScrollbar(ComputedStyle* aStyle) {
-  return aStyle->StyleUI()->HasCustomScrollbars() ||
-         IsScrollbarWidthThin(aStyle);
+// Returns the style for custom scrollbar if the scrollbar part frame should
+// use the custom drawing path, nullptr otherwise.
+//
+// Optionally the caller can pass a pointer to aForDarkBg for whether custom
+// scrollbar may be drawn due to dark background.
+static ComputedStyle* GetCustomScrollbarStyle(nsIFrame* aFrame,
+                                              bool* aDarkScrollbar = nullptr) {
+  ComputedStyle* style = nsLayoutUtils::StyleForScrollbar(aFrame);
+  if (style->StyleUI()->HasCustomScrollbars()) {
+    return style;
+  }
+  bool useDarkScrollbar = !StaticPrefs::widget_disable_dark_scrollbar() &&
+                          nsNativeTheme::IsDarkBackground(aFrame);
+  if (useDarkScrollbar || IsScrollbarWidthThin(style)) {
+    if (aDarkScrollbar) {
+      *aDarkScrollbar = useDarkScrollbar;
+    }
+    return style;
+  }
+  return nullptr;
 }
 
 NS_IMETHODIMP
@@ -1530,10 +1547,9 @@ nsNativeThemeWin::DrawWidgetBackground(gfxContext* aContext, nsIFrame* aFrame,
   }
 
   if (IsWidgetScrollbarPart(aAppearance)) {
-    ComputedStyle* style = nsLayoutUtils::StyleForScrollbar(aFrame);
-    if (ShouldDrawCustomScrollbar(style)) {
-      return DrawCustomScrollbarPart(aContext, aFrame, style, aAppearance,
-                                     aRect, aDirtyRect);
+    if (MayDrawCustomScrollbarPart(aContext, aFrame, aAppearance, aRect,
+                                   aDirtyRect)) {
+      return NS_OK;
     }
   }
 
@@ -2659,8 +2675,7 @@ nsITheme::ThemeGeometryType nsNativeThemeWin::ThemeGeometryTypeForWidget(
 nsITheme::Transparency nsNativeThemeWin::GetWidgetTransparency(
     nsIFrame* aFrame, StyleAppearance aAppearance) {
   if (IsWidgetScrollbarPart(aAppearance)) {
-    ComputedStyle* style = nsLayoutUtils::StyleForScrollbar(aFrame);
-    if (ShouldDrawCustomScrollbar(style)) {
+    if (ComputedStyle* style = GetCustomScrollbarStyle(aFrame)) {
       auto* ui = style->StyleUI();
       if (ui->mScrollbarColor.IsAuto() ||
           ui->mScrollbarColor.AsColors().track.MaybeTransparent()) {
@@ -4158,9 +4173,17 @@ static nscolor AdjustScrollbarFaceColor(nscolor aFaceColor,
 }
 
 // This tries to draw a Windows 10 style scrollbar with given colors.
-nsresult nsNativeThemeWin::DrawCustomScrollbarPart(
-    gfxContext* aContext, nsIFrame* aFrame, ComputedStyle* aStyle,
-    StyleAppearance aAppearance, const nsRect& aRect, const nsRect& aClipRect) {
+bool nsNativeThemeWin::MayDrawCustomScrollbarPart(gfxContext* aContext,
+                                                  nsIFrame* aFrame,
+                                                  StyleAppearance aAppearance,
+                                                  const nsRect& aRect,
+                                                  const nsRect& aClipRect) {
+  bool darkScrollbar = false;
+  ComputedStyle* style = GetCustomScrollbarStyle(aFrame, &darkScrollbar);
+  if (!style) {
+    return false;
+  }
+
   EventStates eventStates = GetContentState(aFrame, aAppearance);
 
   gfxContextAutoSaveRestore autoSave(aContext);
@@ -4172,11 +4195,12 @@ nsresult nsNativeThemeWin::DrawCustomScrollbarPart(
   gfxRect rect =
       ThebesRect(LayoutDevicePixel::FromAppUnits(aRect, p2a).ToUnknownRect());
 
-  const nsStyleUI* ui = aStyle->StyleUI();
+  const nsStyleUI* ui = style->StyleUI();
   auto* customColors =
       ui->mScrollbarColor.IsAuto() ? nullptr : &ui->mScrollbarColor.AsColors();
-  nscolor trackColor = customColors ? customColors->track.CalcColor(*aStyle)
-                                    : NS_RGB(240, 240, 240);
+  nscolor trackColor = customColors ? customColors->track.CalcColor(*style)
+                                    : (darkScrollbar ? NS_RGBA(20, 20, 25, 77)
+                                                     : NS_RGB(240, 240, 240));
   switch (aAppearance) {
     case StyleAppearance::ScrollbarHorizontal:
     case StyleAppearance::ScrollbarVertical:
@@ -4184,7 +4208,7 @@ nsresult nsNativeThemeWin::DrawCustomScrollbarPart(
       ctx->SetColor(Color::FromABGR(trackColor));
       ctx->Rectangle(rect);
       ctx->Fill();
-      return NS_OK;
+      return true;
     }
     default:
       break;
@@ -4211,8 +4235,10 @@ nsresult nsNativeThemeWin::DrawCustomScrollbarPart(
   switch (aAppearance) {
     case StyleAppearance::ScrollbarthumbVertical:
     case StyleAppearance::ScrollbarthumbHorizontal: {
-      nscolor faceColor = customColors ? customColors->thumb.CalcColor(*aStyle)
-                                       : NS_RGB(205, 205, 205);
+      nscolor faceColor = customColors
+                              ? customColors->thumb.CalcColor(*style)
+                              : (darkScrollbar ? NS_RGBA(249, 249, 250, 102)
+                                               : NS_RGB(205, 205, 205));
       faceColor = AdjustScrollbarFaceColor(faceColor, eventStates);
       ctx->SetColor(Color::FromABGR(faceColor));
       ctx->Rectangle(bgRect);
@@ -4276,7 +4302,7 @@ nsresult nsNativeThemeWin::DrawCustomScrollbarPart(
     default:
       MOZ_ASSERT_UNREACHABLE("Unknown widget type");
   }
-  return NS_OK;
+  return true;
 }
 
 ///////////////////////////////////////////

widget/windows/nsNativeThemeWin.h

@@ -105,11 +105,9 @@ class nsNativeThemeWin : private nsNativeTheme, public nsITheme {
                                   StyleAppearance aAppearance);
   void DrawCheckedRect(HDC hdc, const RECT& rc, int32_t fore, int32_t back,
                        HBRUSH defaultBack);
-  nsresult DrawCustomScrollbarPart(gfxContext* aContext, nsIFrame* aFrame,
-                                   mozilla::ComputedStyle* aStyle,
-                                   StyleAppearance aAppearance,
-                                   const nsRect& aRect,
-                                   const nsRect& aClipRect);
+  bool MayDrawCustomScrollbarPart(gfxContext* aContext, nsIFrame* aFrame,
+                                  StyleAppearance aAppearance,
+                                  const nsRect& aRect, const nsRect& aClipRect);
   uint32_t GetWidgetNativeDrawingFlags(StyleAppearance aAppearance);
   int32_t StandardGetState(nsIFrame* aFrame, StyleAppearance aAppearance,
                            bool wantFocused);