[FTF-431] fix: prefetch page Markdown content for better cross-browser support

[jamiehenson]

"Copy as markdown" uses the modern navigator API to copy contents to the clipboard. This works poorly with asynchronous operations (i.e. in this case where we fetched markdown content and then copied it to clipboard) as it breaks the terms of what is considered a user interaction - and Safari is the most strict on this.

The solution here is suboptimal, but save for programatically creating a text area and then having the user click it (a previous approach), this PR makes the operation synchronous by fetching the markdown copy on load and then copying it to the clipboard without async. Naturally this is wasteful if only 1% of people click the markdown copy button, but the payload is very small and it allows for a smoother copy experience. If this is unsatisfactory my suggestion would be to drop the copy and just have the view link.

Tests have been added for PageHeader to represent this.

Also includes a very quick fix for the LeftSidebar overflow property (was getting horizontal scrolls on the bar)

To test: go here in Chrome and Safari and copying should work gracefully as elsewhere.

[coderabbitai]

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fallback-copy

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[jamiehenson]

Lets us abort the fetch if dismounted, was playing with the tests a bit (and is a positive step)

[jamiehenson]

I've made the choice to conditionally hide the markdown buttons if there's no content, since we now grab it ahead-of-time

[jamiehenson]

@kennethkalmer Taken another swing at this (PR updated and puffed out a bit with tests), was an interesting problem. Sorry for the delay on it, the tests were a nightmare.

The main difference is that using navigator.clipboard.writeText() is fine in Safari, but only synchronously, which is why it works for api key selectors and the like where the data is already there to copy. My latest approach is to fetch the markdown content on load, which is more wasteful, but allows it to be done sync not async.

Given the tiny payload it seems like a lesser evil (and also allows a runtime check to see if we should render the markdown buttons at all).

Ready for a look at a point of convenience.

[kennethkalmer]

@jamiehenson thanks, this is a good compromise for the moment and a fantastic learning! I think in the new year we could enhance it with a bit of "feature detection" and then only prefetch the markdown if async clipboard writing is not supported. But for now this creates equal access for everyone, thank you!