I've built Theia without @theia/preview package. Installed markdown-language-features extension [1]. Preview works, but auto-completion doesn't, and there are not any errors in console.
BTW, auto-completion doesn't works in markdown files even with @theia/preview.
[1] https://www.npmjs.com/package/@theia/vscode-builtin-markdown-language-features
@tolusha what type of autocompletion are you looking for in markdown files?
I tried VS Code (reloaded with external extensions disabled), and don't see any completion either.
@vince-fugnitto

@tolusha Do you have both @theia/vscode-builtin-markdown-language-features and @theia/vscode-builtin-markdown without @theia/textmate-grammars? It could be that it is snippet which is not contributed by markdown-language-features.
@akosyakov
You are right! Thank you!