Qt WebAssembly clipboard

Clipboard use on desktop platforms is ubiquitous. Most people use it without thinking. Copy, Paste, and Cut keyboard strokes are in-grained into muscle memory. 

On the web, it can present security issues as someone could read or write to your clipboard without you knowing.

Up until now, Qt for WebAssembly's clipboard was text-only and only within the app itself. Qt 6.3 will have better clipboard support between host and app but also adds copy/pasting of images.

WebAssembly is a sandboxed platform like javascript. There are some extra security hurdles in doing some common things such as copy and paste of binary data such as images. One issue is clipboard use between the host platform and the browser sandbox. Allowing the web app to have access to the clipboard in which it could send arbitrary data without the user knowing could be dangerous for the user.

Browsers generally allow clipboard during user-generated events such as when a user makes common key sequences such as [ctrl | command] c - the copy keys.

Qt itself has support for programmatically copying text and binary data such as images and works great on the desktop, but it presents issues for web browsers. There are workarounds, like using a hidden javascript element and the javascript function execCommand to "copy". However, this function has been depreciated. 

By using the asynchronous Clipboard API and making use of javascript clipboard events where possible, we can bring image clipboard support to Qt WebAssembly. The Clipboard API requires a secure context (https) for full feature use. Among other things, the Clipboard API allows image and arbitrary data to be copied and pasted. Whereas before, only text mime types were supported. 

This API is of course implemented in the different browsers in different ways. On Firefox, read() and write() are only partially implemented and are hidden behind about:config settings. As well, copy/paste of arbitrary binary data does not seem to be supported and mostly silently fails.

Here are the ways browsers support Clipboard API:


  • write() is available without permission in secure contexts and browser extensions, but only from user-initiated event callbacks.
  • Clipboard API
    • secure context (https or localhost)
    • dom.events.asyncClipboard
    • dom.events.asyncClipboard.clipboardItem
    • dom.events.asyncClipboard.read
    • dom.events.testing.asyncClipboard
  • Clipboard API
    • secure context    
  • Clipboard API
    • secure context
    • user permissions
  • read() only supports
    • text/html
    • text/png
Also included in the now merged commit f0be152896471aa392bb1b2b649b66feb31480cc  is a clipboard manual test app that can be used on both desktop and webassembly to test clipboard use.
You can use the clipboard without a secure https context, but you won't get interaction between the host and web app.
This article originally appeared on the authors' blog: http://qtandeverything.blogspot.com/2022/01/qt-webassembly-clipboard.html

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