improve scrolling performance

[x1B]

The details layer currently uses CSS overflow-y: scroll to present long content.

This causes performance issues, primarily under Safari and particularly when painting the details is expensive (e.g. large images).

Plan to fix this:

• do not try to constrain the layer to match the window, instead expand to fit the details contents,
• when opening a popup, the scrollable region of the containing page must be constrained to the popup contents,
• the close button must remain fixed to the top/right corner of the screen,
• when closing the layer, the scroll position must be restored.