Add a banner to let people know they are inside Via

[jon-betts]

Similar to:

• Add a modal dialog to let people know they're in Via

Currently it's quite hard to tell you are inside Via which expands the possibilities of using it for malicious ends. A banner of some sort would:

• Provide a place to suggest people use the safer plugin
• Let them know they aren't on the original site
• Provide a place to let people report bad links

[seanh]

If you're on a https://via.hypothes.is/example.com page there's also no link back to the https://via.hypothes.is/ so that you can proxy another page. A banner could provide that

[seanh]

A banner could also contain a link to our abuse policy and form for reporting URLs that should be blocked