In IJ 2026.2 the MarkdownJCEFHtmlPanel is removed and looking on Google is replaced by a Compose version, do we have any documentation on how to integrate the compose version in plugin, without convert the full plugin in Compose.
It is definitely not removed, and used as before. The only change is that the layout of the plugin is different, com.intellij.markdown.jcef.preview.MarkdownJCEFHtmlPanel belongs to intellij.markdown.jcef module. If you use plugin v2 format, you would need this module in dependencies of a plugin.
I also do not recommend depending on MarkdownJCEFHtmlPanel as it is more like implementation detail than API.