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FirefoxPWA
Discontinued A tool to install, manage and use Progressive Web Apps (PWAs) in Mozilla Firefox [Moved to: https://github.com/filips123/PWAsForFirefox]
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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.
I have been use Edge's PWA, web catalog
Native menus (and context menus) are available by default. https://www.electronjs.org/docs/api/menu In Linux, it uses GTK at places (file picker etc.), but npm repository usually have packages for KDE. For your question regarding calling native APIs - here's an example source code of a very small project (not mine). https://github.com/drakmail/electron-native-code. You can see how they call RequestReview swift API from inside the JavaScript file. In this instance, an Objective-C module.
Seems like Signal uses this:https://github.com/signalapp/Signal-Desktop/blob/1032249b82003ea357a67ba505734079d7bf7888/main.js