Function framed_in_browser 

pub(crate) fn framed_in_browser(rig: &mut Rig<'_>, zoom: Option<f32>)

The browser’s canvas, framed at zoom about the sheet’s middle — or the whole sheet fitted (the 10% floor) when None — checked to land there. A framing pads the rect it is given, so the rect is corrected by the zoom the last framing landed at.