[UI-side compositing] NSScrollerImps are still created in the web process (triggering CATransactions)

https://bugs.webkit.org/show_bug.cgi?id=258369
rdar://109689491

Reviewed by Simon Fraser.

With UI-side compositing, NSScrollerImps are now kept in the UI-process for all scrollbars
except for RenderListBox scrollbars. To prevent the unnecessary creation of NSScrollerImps
in the web process in ScrollbarThemeMac::registerScrollbar, check if the scrollbar is one
of a RenderListBox and if we have a RemoteScrollbarsController.

* Source/WebCore/platform/ScrollableArea.cpp:
(WebCore::ScrollableArea::shouldRegisterScrollbars const):
* Source/WebCore/platform/ScrollableArea.h:
* Source/WebCore/platform/Scrollbar.cpp:
(WebCore::Scrollbar::shouldRegisterScrollbar const):
* Source/WebCore/platform/Scrollbar.h:
* Source/WebCore/platform/ScrollbarsController.h:
(WebCore::ScrollbarsController::shouldRegisterScrollbars const):
* Source/WebCore/platform/mac/ScrollbarThemeMac.mm:
(WebCore::ScrollbarThemeMac::registerScrollbar):
* Source/WebKit/WebProcess/WebPage/RemoteLayerTree/RemoteScrollbarsController.h:

Canonical link: https://commits.webkit.org/265731@main

Author: nmoucht

Source/WebCore/platform/Scrollbar.cpp

@@ -528,4 +528,9 @@ float Scrollbar::deviceScaleFactor() const
     return m_scrollableArea.deviceScaleFactor();
 }
 
+bool Scrollbar::shouldRegisterScrollbar() const
+{
+    return m_scrollableArea.scrollbarsController().shouldRegisterScrollbars();
+}
+
 } // namespace WebCore

Source/WebCore/platform/Scrollbar.h

@@ -141,6 +141,8 @@ class Scrollbar : public Widget {
 
     float deviceScaleFactor() const;
 
+    bool shouldRegisterScrollbar() const;
+
 protected:
     Scrollbar(ScrollableArea&, ScrollbarOrientation, ScrollbarWidth, ScrollbarTheme* = nullptr, bool isCustomScrollbar = false);
 

Source/WebCore/platform/ScrollbarsController.h

@@ -94,6 +94,7 @@ class ScrollbarsController {
     WEBCORE_EXPORT virtual void setScrollbarVisibilityState(ScrollbarOrientation, bool) { }
 
     WEBCORE_EXPORT virtual bool shouldDrawIntoScrollbarLayer(Scrollbar&) const { return true; }
+    WEBCORE_EXPORT virtual bool shouldRegisterScrollbars() const { return true; }
 
 private:
     ScrollableArea& m_scrollableArea;

Source/WebCore/platform/mac/ScrollbarThemeMac.mm

@@ -156,7 +156,7 @@ + (void)registerAsObserver
 
 void ScrollbarThemeMac::registerScrollbar(Scrollbar& scrollbar)
 {
-    if (scrollbar.isCustomScrollbar())
+    if (scrollbar.isCustomScrollbar() || !scrollbar.shouldRegisterScrollbar())
         return;
 
     bool isHorizontal = scrollbar.orientation() == ScrollbarOrientation::Horizontal;

Source/WebKit/WebProcess/WebPage/RemoteLayerTree/RemoteScrollbarsController.h

@@ -51,6 +51,7 @@ class RemoteScrollbarsController final : public WebCore::ScrollbarsController {
 
     void setScrollbarVisibilityState(WebCore::ScrollbarOrientation, bool) final;
     bool shouldDrawIntoScrollbarLayer(WebCore::Scrollbar&) const final;
+    bool shouldRegisterScrollbars() const final { return scrollableArea().isListBox(); }
 
 private:
     bool m_horizontalOverlayScrollbarIsVisible { false };