vscode.dev: Make UI more stable when opening a PR URL

[egamma]

Open a PR from the navbar, e.g. https://vscode.dev/github/microsoft/vscode/pull/124803

Observe the activity bar and notice the appearance of:

• the Explorer view
• the GHPRI view
• the Explorer view
• the GHPRI view

[alexr00]

Turns out this is coming from GHPRI. There was some flow added 9 months ago that aimed to improve the perceived performance of loading all the views. There is a gap between when views are shown and hidden, which causes the Explorer view to show in-between.