Bug 1735797 - Fix scrollbar caching on macOS. r=mstange

For that:

 * Remove support for caching resizer styles, since they don't seem
   common enough to warrant it, unlike all other scrollbar parts.

 * Make scrollcorners have pointer-events: none with overlay scrollbars,
   so that the setup works / we can assert that if we have overlay
   scrollbars then we don't care about the pointer-events value on
   ancestors.

Differential Revision: https://phabricator.services.mozilla.com/D128772

layout/generic/AnonymousContentKey.h

@@ -32,22 +32,13 @@ enum class AnonymousContentKey : uint8_t {
 
   // all
   Type_ScrollCorner              = 0x01,
-  Type_Resizer                   = 0x02,
-  Type_Scrollbar                 = 0x03,
-  Type_ScrollbarButton           = 0x04,
-  Type_Slider                    = 0x05,
+  Type_Scrollbar                 = 0x02,
+  Type_ScrollbarButton           = 0x03,
+  Type_Slider                    = 0x04,
 
   // scrollbar, scrollbarbutton, slider
   Flag_Vertical                  = 0x08,
 
-  // resizer
-  // 3-bit bitfield in 0x38 storing dir="" and flip=""
-  Flag_Resizer_Right             = 0x08,
-  Flag_Resizer_Bottom            = 0x10,
-  Flag_Resizer_BottomLeft        = 0x18,
-  Flag_Resizer_BottomRight       = 0x20,
-  Flag_Resizer_Bottom_Flip       = 0x28,
-
   // scrollbarbutton
   Flag_ScrollbarButton_Down      = 0x10,
   Flag_ScrollbarButton_Bottom    = 0x20,

layout/generic/nsGfxScrollFrame.cpp

@@ -5440,8 +5440,6 @@ nsresult ScrollFrameHelper::CreateAnonymousContent(
   if (neededAnonContent.contains(AnonymousContentType::Resizer)) {
     MOZ_ASSERT(!mIsRoot, "Root scroll frame shouldn't be resizable");
 
-    AnonymousContentKey key = AnonymousContentKey::Type_Resizer;
-
     RefPtr<NodeInfo> nodeInfo;
     nodeInfo = nodeInfoManager->GetNodeInfo(
         nsGkAtoms::resizer, nullptr, kNameSpaceID_XUL, nsINode::ELEMENT_NODE);
@@ -5454,7 +5452,6 @@ nsresult ScrollFrameHelper::CreateAnonymousContent(
       case StyleResize::Horizontal:
         if (IsScrollbarOnRight()) {
           dir.AssignLiteral("right");
-          key |= AnonymousContentKey::Flag_Resizer_Right;
         } else {
           dir.AssignLiteral("left");
         }
@@ -5464,18 +5461,13 @@ nsresult ScrollFrameHelper::CreateAnonymousContent(
         if (!IsScrollbarOnRight()) {
           mResizerContent->SetAttr(kNameSpaceID_None, nsGkAtoms::flip, u""_ns,
                                    false);
-          key |= AnonymousContentKey::Flag_Resizer_Bottom_Flip;
-        } else {
-          key |= AnonymousContentKey::Flag_Resizer_Bottom;
         }
         break;
       case StyleResize::Both:
         if (IsScrollbarOnRight()) {
           dir.AssignLiteral("bottomright");
-          key |= AnonymousContentKey::Flag_Resizer_BottomRight;
         } else {
           dir.AssignLiteral("bottomleft");
-          key |= AnonymousContentKey::Flag_Resizer_BottomLeft;
         }
         break;
       default:
@@ -5486,7 +5478,7 @@ nsresult ScrollFrameHelper::CreateAnonymousContent(
                              u"_parent"_ns, false);
     mResizerContent->SetAttr(kNameSpaceID_None, nsGkAtoms::clickthrough,
                              u"always"_ns, false);
-    aElements.AppendElement(ContentInfo(mResizerContent, key));
+    aElements.AppendElement(mResizerContent);
   }
 
   if (neededAnonContent.contains(AnonymousContentType::HorizontalScrollbar) &&

layout/style/res/scrollbars.css

@@ -31,6 +31,9 @@ scrollbar[root="true"] {
     pointer-events: none;
     opacity: 0;
   }
+  scrollcorner {
+    pointer-events: none;
+  }
 }
 
 slider {