Bug 1812868 - Tweak RDM and theme setup. r=mstange

This is in order to make nsPresContext::UseOverlayScrollbars() thread-safe for
stylo usage.

Document::GetBrowsingContext() does ref-counting of main-thread-only
objects, and we don't want that.

Similarly nsPresContext::Theme() mutates the pres context which is not
something we can do from multiple threads.

Reviewed in: https://phabricator.services.mozilla.com/D168148

docshell/base/BrowsingContext.cpp

@@ -2757,21 +2757,7 @@ void BrowsingContext::DidSet(FieldIndex<IDX_InRDMPane>, bool aOldValue) {
   if (GetInRDMPane() == aOldValue) {
     return;
   }
-
-  PreOrderWalk([&](BrowsingContext* aContext) {
-    if (nsIDocShell* shell = aContext->GetDocShell()) {
-      if (nsPresContext* pc = shell->GetPresContext()) {
-        pc->RecomputeTheme();
-
-        // This is a bit of a lie, but this affects the overlay-scrollbars
-        // media query and it's the code-path that gets taken for regular system
-        // metrics changes via ThemeChanged().
-        pc->MediaFeatureValuesChanged(
-            {MediaFeatureChangeReason::SystemMetricsChange},
-            MediaFeatureChangePropagation::JustThisDocument);
-      }
-    }
-  });
+  PresContextAffectingFieldChanged();
 }
 
 bool BrowsingContext::CanSet(FieldIndex<IDX_PageAwakeRequestCount>,

layout/base/nsPresContext.cpp

@@ -278,6 +278,7 @@ nsPresContext::nsPresContext(dom::Document* aDocument, nsPresContextType aType)
       mFontFeatureValuesDirty(true),
       mFontPaletteValuesDirty(true),
       mIsVisual(false),
+      mInRDMPane(false),
       mHasWarnedAboutTooLargeDashedOrDottedRadius(false),
       mQuirkSheetAdded(false),
       mHadNonBlankPaint(false),
@@ -858,6 +859,8 @@ void nsPresContext::AttachPresShell(mozilla::PresShell* aPresShell) {
   // have a presshell, and hence a document.
   GetUserPreferences();
 
+  EnsureTheme();
+
   nsIURI* docURI = doc->GetDocumentURI();
 
   if (IsDynamic() && docURI) {
@@ -933,6 +936,8 @@ void nsPresContext::RecomputeBrowsingContextDependentData() {
     return browsingContext->GetEmbedderColorSchemes().mPreferred;
   }());
 
+  SetInRDMPane(top->GetInRDMPane());
+
   if (doc == mDocument) {
     // Medium doesn't apply to resource documents, etc.
     RefPtr<nsAtom> mediumToEmulate;
@@ -1286,6 +1291,14 @@ void nsPresContext::UpdateEffectiveTextZoom() {
       MediaFeatureChangePropagation::JustThisDocument);
 }
 
+void nsPresContext::SetInRDMPane(bool aInRDMPane) {
+  if (mInRDMPane == aInRDMPane) {
+    return;
+  }
+  mInRDMPane = aInRDMPane;
+  RecomputeTheme();
+}
+
 float nsPresContext::GetDeviceFullZoom() {
   return mDeviceContext->GetFullZoom();
 }
@@ -1626,11 +1639,15 @@ void nsPresContext::RecordInteractionTime(InteractionType aType,
   }
 }
 
-nsITheme* nsPresContext::EnsureTheme() {
+nsITheme* nsPresContext::Theme() const {
+  MOZ_ASSERT(mTheme);
+  return mTheme;
+}
+
+void nsPresContext::EnsureTheme() {
   MOZ_ASSERT(!mTheme);
   if (Document()->ShouldAvoidNativeTheme()) {
-    BrowsingContext* bc = Document()->GetBrowsingContext();
-    if (bc && bc->Top()->InRDMPane()) {
+    if (mInRDMPane) {
       mTheme = do_GetRDMThemeDoNotUseDirectly();
     } else {
       mTheme = do_GetBasicNativeThemeDoNotUseDirectly();
@@ -1639,7 +1656,6 @@ nsITheme* nsPresContext::EnsureTheme() {
     mTheme = do_GetNativeThemeDoNotUseDirectly();
   }
   MOZ_RELEASE_ASSERT(mTheme);
-  return mTheme;
 }
 
 void nsPresContext::RecomputeTheme() {
@@ -1651,17 +1667,21 @@ void nsPresContext::RecomputeTheme() {
   if (oldTheme == mTheme) {
     return;
   }
-  // Theme only affects layout information, not style, so we just need to
-  // reframe (as it affects whether we create scrollbar buttons for example).
-  RebuildAllStyleData(nsChangeHint_ReconstructFrame, RestyleHint{0});
+  // Theme affects layout information (as it affects whether we create
+  // scrollbar buttons for example) and also style (affects the
+  // scrollbar-inline-size env var).
+  RebuildAllStyleData(nsChangeHint_ReconstructFrame,
+                      RestyleHint::RecascadeSubtree());
+  // This is a bit of a lie, but this affects the overlay-scrollbars
+  // media query and it's the code-path that gets taken for regular system
+  // metrics changes via ThemeChanged().
+  MediaFeatureValuesChanged({MediaFeatureChangeReason::SystemMetricsChange},
+                            MediaFeatureChangePropagation::JustThisDocument);
 }
 
 bool nsPresContext::UseOverlayScrollbars() const {
-  if (LookAndFeel::GetInt(LookAndFeel::IntID::UseOverlayScrollbars)) {
-    return true;
-  }
-  BrowsingContext* bc = Document()->GetBrowsingContext();
-  return bc && bc->Top()->InRDMPane();
+  return LookAndFeel::GetInt(LookAndFeel::IntID::UseOverlayScrollbars) ||
+         mInRDMPane;
 }
 
 void nsPresContext::ThemeChanged(widget::ThemeChangeKind aKind) {

layout/base/nsPresContext.h

@@ -573,6 +573,7 @@ class nsPresContext : public nsISupports, public mozilla::SupportsWeakPtr {
   }
   void SetFullZoom(float aZoom);
   void SetOverrideDPPX(float);
+  void SetInRDMPane(bool aInRDMPane);
 
  public:
   float GetFullZoom() { return mFullZoom; }
@@ -824,18 +825,7 @@ class nsPresContext : public nsISupports, public mozilla::SupportsWeakPtr {
    */
   uint32_t GetBidi() const;
 
-  /*
-   * Obtain a native theme for rendering our widgets (both form controls and
-   * html)
-   *
-   * Guaranteed to return non-null.
-   */
-  nsITheme* Theme() MOZ_NONNULL_RETURN {
-    if (MOZ_LIKELY(mTheme)) {
-      return mTheme;
-    }
-    return EnsureTheme();
-  }
+  nsITheme* Theme() const MOZ_NONNULL_RETURN;
 
   void RecomputeTheme();
 
@@ -1352,6 +1342,9 @@ class nsPresContext : public nsISupports, public mozilla::SupportsWeakPtr {
 
   unsigned mIsVisual : 1;
 
+  // Are we in the RDM pane?
+  unsigned mInRDMPane : 1;
+
   unsigned mHasWarnedAboutTooLargeDashedOrDottedRadius : 1;
 
   // Have we added quirk.css to the style set?
@@ -1383,7 +1376,7 @@ class nsPresContext : public nsISupports, public mozilla::SupportsWeakPtr {
 
   void LastRelease();
 
-  nsITheme* EnsureTheme();
+  void EnsureTheme();
 
 #ifdef DEBUG
  private: