Custom Scrollbar API Review (#4073) (#4221)

Custom ScrollBar API review done

Author: Diana Qu

specs/CustomScrollbar.md

@@ -0,0 +1,95 @@
+Custom ScrollBar Style
+===
+
+# Background
+Developers would like to be able to set the ScrollBar to Windows fluent overlay style and 
+match the Windows theme. Some customers have used browser flags but they change names and behavior and are not reliable. Thus we want
+to provide an extendable API that allows developers to set ScrollBar style. Currently, 
+only Windows fluent overlay are supported, adding new styles in the future will not introduce
+any breaking changes.
+
+# Examples
+## WinRT and .NET   
+```c#
+// Create WebView Environment with option
+async void CreateEnvironmentWithOption()
+{
+    CoreWebView2EnvironmentOptions options = new CoreWebView2EnvironmentOptions();
+    options.ScrollBarStyle = ScrollBarStyle.FluentOverlay;
+    CoreWebView2Environment environment = await CoreWebView2Environment.CreateAsync(options: options);
+    webview.EnsureCoreWebView2Async(environment);
+}
+```
+
+## Win32 C++
+```cpp
+void AppWindow::InitializeWebView()
+{
+    auto options = Microsoft::WRL::Make<CoreWebView2EnvironmentOptions>();
+    COREWEBVIEW2_SCROLLBAR_STYLE style = COREWEBVIEW2_SCROLLBAR_STYLE_FLUENT_OVERLAY;
+    CHECK_FAILURE(options->put_ScrollBarStyle(style));
+
+    // ... other option properties
+
+    // ... CreateCoreWebView2EnvironmentWithOptions
+}
+```
+
+# API Notes
+
+See [API Details](#api-details) section below for API reference.
+
+# API Details
+## Win32 C++
+
+```IDL
+interface ICoreWebView2EnvironmentOptions7;
+
+[v1_enum]
+typedef enum COREWEBVIEW2_SCROLLBAR_STYLE {
+  /// Browser default ScrollBar style
+  COREWEBVIEW2_SCROLLBAR_STYLE_DEFAULT,
+  /// Window style fluent overlay scroll bar
+  /// Please see [Fluent UI](https://developer.microsoft.com/en-us/fluentui#/)
+  /// for more details on fluent UI.
+  COREWEBVIEW2_SCROLLBAR_STYLE_FLUENT_OVERLAY
+} COREWEBVIEW2_SCROLLBAR_STYLE;
+
+/// Additional options used to create WebView2 Environment.
+[uuid(9c8ac95a-6b5f-4efb-b5f6-98bb33469759), object, pointer_default(unique)]
+interface ICoreWebView2EnvironmentOptions7 : ICoreWebView2EnvironmentOptions6 { 
+  /// Get the ScrollBar style being set on the WebView2 Environment.
+  [propget] HRESULT ScrollBarStyle([out, retval] COREWEBVIEW2_SCROLLBAR_STYLE* value);
+  /// Set ScrollBar style to be used. The default is `COREWEBVIEW2_SCROLLBAR_STYLE_DEFAULT`
+  /// which specifies the default browser ScrollBar style.
+  /// CSS styles that modify the ScrollBar applied on top of native ScrollBar styling 
+  /// that is selected with `ScrollBarStyle`.
+  [propput] HRESULT ScrollBarStyle([in] COREWEBVIEW2_SCROLLBAR_STYLE value);
+}
+```
+
+## .NET and WinRT
+
+```c#
+namespace Microsoft.Web.WebView2.Core
+{
+
+    enum ScrollBarStyle
+    {
+        Default = 0,
+        FluentOverlay = 1,
+    };
+    
+    // ...
+    runtimeclass CoreWebView2EnvironmentOptions
+    {
+        [interface_name("Microsoft.Web.WebView2.Core.ICoreWebView2EnvironmentOptions7")]
+        {
+            // Set ScrollBar style
+            ScrollBarStyle ScrollBarStyle { get; set; };
+        }
+    }
+
+    // ...
+}
+```