Submenus in TreeView overflow and pierce through when horizontal space is insufficient

[yiliang114]

When the horizontal width of the TreeView is too narrow, its submenus overflow and visually "pierce" through the container boundaries, causing layout issues.

Version: 1.103.2
Commit: 6f17636
User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/139.0.0.0 Safari/537.36
Embedder: vscode.dev

Steps to Reproduce:

1. Open a workspace with a folder that contains deeply nested paths or files with long names.
2. Right-click an item several levels deep in the Explorer TreeView.
3. Observe that the context menu extends past the right edge of the VS Code window;

[yiliang114]

VS Code uses two distinct ways to render drop-down/context menus:

1. Shadow-DOM mode (used for view/title menus)
   • Created via WorkbenchToolBar with renderDropdownAsChildElement = true
   • The menu element is attached inside the same shadow root as the host component.
   • It therefore picks up the component-local z-index defined in the shadow tree (FIXED_SHADOW positioning).

2. Plain-DOM mode (used for view/item/context menus)
   • Created through IContextMenuService.showContextView, which does not pass domForShadowRoot.
   • The menu is appended to the document body as a regular absolutely-positioned element.
   • It never sees the Shadow-DOM style sheet that contains the z-index rule, so the rule has no effect.

Because the TreeView right-click menu is rendered in plain-DOM mode, its z-index is governed only by global styles. When labels are long the menu keeps expanding rightward and can overflow the viewport, whereas the Shadow-DOM menus are correctly clipped by their stacking context.

I believe the overflow happens because the context menus opened from a TreeView are created in the normal DOM rather than inside the component’s shadow root. The shadow styles—including the z-index that should constrain the menu—are therefore not applied, so the menu is free to grow beyond the window edge

[yiliang114]

hi @sbatten Can you take time to look at this issue and PR?

[yiliang114]

In fact, my changes are also inspired by claude4. #263555