Yes this is a regression in Insiders:

Here the behaviour in Stable:

Broken by bfe83a8 🤔 cc @jrieken

Turns out there is a different between TPromise.join and Promise.all, very similar to the difference between promise ctors:

• In the TPromise version, the join then callback gets called as soon as signalInnerHTML fires. When renderMarkdown returns, all colorized code is in the hover DOM. The hover widget tells the scrollbar to scan the DOM to compute the content height and everything works fine.
• In the Promise version, the join then callback only gets called on the next execution frame... so the hover widget tells the scrollbar to scan the DOM before any of the colorized code is placed in the DOM.

As a fix for this particular issue, I've added a codeBlockRenderCallback option to the markdown render function, which gets called every time colorised code gets inserted into the DOM. Ugly, but effective. @mjbvz

Thanks @joaomoreno! That approach looks like it will work.

The whole reason these are async is the call to getOrCreateMode. Not sure if that can be avoided