onDidChangeActiveTextEditor getting fired with "plaintext" document when opening new file

[mjbvz]

Steps to Reproduce:

1. In a repo with 3 markdown files.
2. Make sure all files are closed and then restart vscode
3. Open the first markdown file and open the preview to the side
4. With the markdown editor focused, click on one of the other markdown files in the explorer

Bug
Markdown preview does not update to show the preview of the new document.

The root cause is that onDidChangeActiveTextEditor is fired with a editor.document.languageId that is set to plaintext instead of markdown

Does this issue occur when all extensions are disabled?: Yes

[mjbvz]

Could be a promise conversion timing issue. Not sure who the right owner is here but I did some initial investigation. Here's what I believe is going wrong:

1. Starting in ModelServiceImpl.createModel
2. This method calls setMode which invokes .then on the mode promise. This block is not evaluated yet.
3. We next continue on with createModel which calls into MainThreadDocumentsAndEditors. This code sends the document delta to the extesion host with the incorrect plaintext document mode.
4. At some later point, the .then block from setMode is then evaluated and we update the document to have the correct markdown mode

I suspect the .then in setMode used to by resolved synchronously

[rebornix]

This is not good. Every time when a file is opened, we will warm up the tokens first before displaying, which is a sync task. So what happens now is

• Tokenize up to 50ms with plaintext
• Language update kicks in
• All tokens are thrown away

[bpasero]

Yeah could be likely a Promise/TPromise conversion regression.

[alexdima]

Definitely caused by the asynchronous then, thank you for the analysis @mjbvz. Since I converted the IModeService to use native promises, I will look into addressing the issue.