Slash-command popup overlaps and bleeds through the branch/checkout repo dialog (z-index/layering bug)

### Summary

The slash-command (`/`) autocomplete popup renders **on top of and bleeds through** the "Pick a branch and checkout mode for each repo" dialog. Both layers are visible at the same time — the command list (`/inbox`, `/chronicle`, `/docx`, …) overlaps the repo rows and their branch dropdowns, making both unreadable. Looks like a z-index / layering (modal stacking) issue.

### Steps to reproduce

1. Open the "Pick a branch and checkout mode for each repo" screen (multi-repo branch selection).
2. With that dialog open, focus the prompt box and type `/` to trigger the slash-command menu.
3. The slash-command dropdown appears over the dialog, and the two layers visibly overlap / show through each other.

### Expected

The slash-command menu should either be suppressed while that dialog is open, or render in its own opaque layer that doesn't overlap/bleed through the repo+branch list.

### Actual

Both UIs render in the same space simultaneously — text from the repo list (e.g. "FE.MSecSCC", "WDATP.Infra.App.Tenant…", branch "main" dropdowns) shows through the slash-command popup, and vice versa.

### Environment

- GitHub Copilot desktop app, macOS (Apple Silicon).

### Additional context

Screenshot attached showing the overlap (repo/branch dialog behind, slash-command list in front, both visible through each other).

Commencez par reproduire la boîte de dialogue de sélection de branch pour plusieurs dépôts, puis ouvrez le menu de slash-command depuis la zone de saisie. Inspectez la superposition de la boîte de dialogue et de la popup afin de déterminer si le menu doit être supprimé ou rendu sous forme de calque opaque sans chevauchement ; c'est terminé lorsque les lignes des dépôts, les listes déroulantes de branch et la liste des commandes restent lisibles séparément.

Rédigé par le modèle d'indexation à partir du texte de l'issue.