Hacking the Windows Clipboard: CF_HTML vs CF_RTF

This is Part 2 of the PowerToys Experience series. Read Part 1 here.

I was trying to build an orchestration flow for the Windows Clipboard when I realized most developers treat it like a magical black box. It is not. The Windows Clipboard is a brutal, complex data structure that holds multiple distinct payloads simultaneously. When you hit `Ctrl+V`, the receiving application arbitrarily decides which payload it prefers.

Building "Paste as Rich Text"

My goal was to allow users to copy raw Markdown and paste it as fully-formatted Rich Text. Instead of trying to generate archaic RTF bytes from scratch—a massive architectural vulnerability—I leveraged the existing `Markdig` dependency in the PowerToys repository to compile the Markdown into semantic HTML.

// 1. Compile raw Markdown into Semantic HTML using Markdig
var pipeline = new MarkdownPipelineBuilder().UseAdvancedExtensions().Build();
string htmlContent = Markdown.ToHtml(rawMarkdown, pipeline);

Then came the critical structural bypass: You can't just shove raw HTML into the clipboard. You have to format it using the native HtmlFormatHelper, which wraps the HTML string in a highly specific header containing exact byte offsets so the receiving application knows where the payload starts and ends.

// 2. Wrap the HTML in the CF_HTML byte-offset header
string clipboardHtml = HtmlFormatHelper.CreateHtmlFormat(htmlContent);

// 3. Inject into the Windows Clipboard as CF_HTML
Clipboard.SetText(clipboardHtml, TextDataFormat.Html);

While CF_HTML works flawlessly for 95% of modern apps, true universal support requires generating BOTH CF_HTML and CF_RTF simultaneously. This is the exact architectural debate currently happening on the open-source PR thread.

In the final part of this series, I'll dive into the brutal reality of open-source CI pipelines, how I got a PR rejected, and how I had to rip out 35 seconds of thread delays to save the automated UI tests.

Continue reading the series:
Part 3: The Brutal Reality of Open Source CI Pipelines ➔

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