@slackfmt/clipboard

Native Node.js addon for reading and writing Slack-formatted content to the system clipboard. Built with Rust using napi-rs and clipboard-rs.

API

writeClipboard(plainText: string, deltaText: string): void
readClipboard(): { plainText?: string, deltaText?: string }

• writeClipboard writes plain text and Slack Quill Delta JSON to the clipboard.
• readClipboard reads both back.

The delta text is stored in a format that Slack's desktop app (Electron) can read natively as rich text.

How it works: org.chromium.web-custom-data

The problem

When Slack's web app copies rich text, it calls:

clipboardData.setData("slack/texty", deltaJson);

slack/texty is a custom MIME type. The browser's Clipboard API only maps standard types (text/plain, text/html) directly to OS clipboard entries. For non-standard MIME types like slack/texty, Chromium bundles them into a single binary blob and writes it to the OS clipboard under the type org.chromium.web-custom-data.

The OS clipboard is flexible

Operating systems accept arbitrary clipboard type identifiers:

• macOS: NSPasteboard accepts any string as a pasteboard type
• Windows: RegisterClipboardFormat() registers custom type names
• Linux: X11 atoms / Wayland MIME types can be any string

The limitation isn't the OS -- it's the browser's Clipboard API that restricts how custom MIME types are persisted. Chromium works around this by using org.chromium.web-custom-data as a container for all non-standard types.

The Chromium Pickle format

The org.chromium.web-custom-data blob uses Chromium's "Pickle" binary serialization:

[u32 LE] payload size
[u32 LE] number of entries
For each entry:
  [u32 LE] MIME type character count (UTF-16 code units)
  [bytes]  MIME type as UTF-16LE
  [bytes]  padding to 4-byte alignment
  [u32 LE] value character count (UTF-16 code units)
  [bytes]  value as UTF-16LE
  [bytes]  padding to 4-byte alignment

For slackfmt, the blob contains a single entry: type slack/texty with the Quill Delta JSON as the value.

Why this works

When you paste into Slack's desktop app (which is Electron/Chromium), it reads the org.chromium.web-custom-data blob from the OS clipboard, decodes the Pickle, finds the slack/texty entry, and interprets it as native Quill Delta rich text -- exactly as if you had copied from Slack itself.

The OS clipboard is just a dumb pipe. Both this addon and Slack's Electron app speak the same Chromium Pickle format, so the round-trip works transparently.

References

• The web's clipboard, and how it stores data of different types -- Deep dive into how org.chromium.web-custom-data works and the Pickle binary format
• Web custom formats for the Async Clipboard API -- Official Chrome blog on custom clipboard format support
• Chromium source: DataTransfer.cpp -- Chromium implementation of clipboardData.setData()
• Chromium source: ui/base/clipboard -- Platform-specific clipboard code (macOS, Windows, Linux)
• W3C Clipboard Pickling explainer -- W3C spec discussion on custom clipboard formats
• Chromium issue 487266 -- Original Chromium issue for custom MIME type clipboard support

Building

Requires Rust toolchain and Node.js:

pnpm install
pnpm build       # release build
pnpm build:debug  # debug build (faster compilation)

Platform support

Pre-built binaries are provided for:

• macOS (arm64, x64)
• Windows (x64)
• Linux (x64, arm64)