feat(frontend): render the overlay scrollbar in the config panel

Wires OverlayScrollbar into the playground config pane: the pane becomes
the positioned hover group, and the thumb is rendered as a sibling of
the scroller so it floats over the content.

Author: mmabrouk

web/oss/src/components/Playground/Components/MainLayout/index.tsx

@@ -21,6 +21,7 @@ import AgentChatSkeleton from "@/oss/components/AgentChatSlice/components/AgentC
 import {chatPanelMaximizedAtom} from "@/oss/components/AgentChatSlice/state/panelLayout"
 // Direct file import — the SessionInspector barrel would statically pull the (dynamic,
 // open-on-demand) inspector drawer back into this chunk.
+import OverlayScrollbar from "@/oss/components/OverlayScrollbar"
 import PanelSessionInspectorButton from "@/oss/components/SessionInspector/PanelSessionInspectorButton"
 import {routerAppIdAtom} from "@/oss/state/app/selectors/app"
 import {playgroundEarlyAgentStateAtom} from "@/oss/state/workflow"
@@ -268,7 +269,7 @@ const PlaygroundMainView = ({
     const animateSplit = justToggled || holdAnimate
 
     const variantRefs = useRef<(HTMLDivElement | null)[]>([])
-    const {setConfigPanelRef, setGenerationPanelRef} = usePlaygroundScrollSync({
+    const {configPanelRef, setConfigPanelRef, setGenerationPanelRef} = usePlaygroundScrollSync({
         enabled: isComparisonView,
     })
 
@@ -375,7 +376,7 @@ const PlaygroundMainView = ({
                             The notice lives OUTSIDE the scroller, so it sits at the pane's bottom
                             edge regardless of content height or scroll position. */}
                         <div
-                            className={clsx("flex h-full min-h-0 w-full flex-col", {
+                            className={clsx("group relative flex h-full min-h-0 w-full flex-col", {
                                 // Config = the raised authoring surface (covers the notice too).
                                 "ag-panel-raised": isAgentConfig,
                             })}
@@ -437,6 +438,9 @@ const PlaygroundMainView = ({
                                     )}
                                 </>
                             </section>
+                            {!isComparisonView ? (
+                                <OverlayScrollbar target={configPanelRef} />
+                            ) : null}
                             {!isComparisonView && isAgentConfig && primaryConfigId ? (
                                 <AgentCommitNotice revisionId={primaryConfigId} />
                             ) : null}