Editor `z-index` challenges

[jrieken]

We have seen a couple of bugs related to z-index and it time to (1) spell out all places and (2) assign values to features.

• minimap
• scrollbar slider
• sticky scroll
• zone widgets
• etc ...

[jrieken]

related bugs https://github.com/microsoft/vscode/issues/172944, https://github.com/microsoft/vscode-copilot/issues/8127, https://github.com/microsoft/vscode-internalbacklog/issues/4298, https://github.com/microsoft/vscode/issues/207734, https://github.com/microsoft/vscode/issues/198122, https://github.com/microsoft/vscode/issues/174134