[DataGrid] Fix right column group header border with virtualization (@…

…hendrikpeilke) (#15503)

Co-authored-by: Hendrik Peilke <143817985+hendrikpeilke@users.noreply.github.com>
Co-authored-by: Kenan Yusuf <kenan.m.yusuf@gmail.com>
Co-authored-by: Andrew Cherniavskii <andrew@mui.com>

packages/x-data-grid/src/hooks/features/columnHeaders/useGridColumnHeaders.tsx

@@ -464,7 +464,7 @@ export const useGridColumnHeaders = (props: UseGridColumnHeadersProps) => {
           pinnedPosition={pinnedPosition}
           style={style}
           indexInSection={indexInSection}
-          sectionLength={rowStructure.length}
+          sectionLength={visibleColumnGroupHeader.length}
           gridHasFiller={gridHasFiller}
         />
       );