Maybe not exactly this feature but I just added something similar to the ProjectOrganizer plugin in https://github.com/geany/geany-plugins/pull/1341:
<img width="575" alt="Screenshot 2024-08-06 at 11 05 56" src="https://github.com/user-attachments/assets/7522f0a1-26d9-4976-9409-da933b0407b9">
It works similarly to vscode - when there's no prefix, it searches files, with the `@` prefix it searches in current document's symbols, with the `#` prefix it searches in all symbols and with the `:` prefix you can jump to the specified line in the current document. You can assign a keybinding for this popup for quick access.
It works independently of other features provided by the plugin and works also when there's no project open.
I'm planning to merge that pull request soon.