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ViewFinderJS
Discontinued :camera: ViewFinder - Remote isolated browser API for security, automation visibility and interactivity. RBI. CBII. Remote browser isolation, embeddable BrowserView, secure chrome-as-a-service. Managed, variable bandwidth and co-browsing options available in Pro versions. Like S2, WebGap, Bromium, Authentic8, Menlo Security and Broadcom, but free and open-source. Integrated secure document viewing with CDR from https://github.com/dosyago/p2%2e [Moved to: https://github.com/i5ik/ViewFinder]
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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
Are you aware of pure-JS in-browser VNC protocol implementations such as http://guacamole.apache.org/ ? This works over websockets, so even encryption is covered. On the server side, run a Xvnc, fullscreen chrome without window manager or decorations. As the OP said, this should give superiour performance, responsiveness and features. And no need for a client side browser plugin.
Another thing you could look into is https://www.brow.sh/ -- they also have a pretty good performance and feature coverage. Maybe there's something your project can learn from them :-)