Discrepancies using vscode.Selection in Theia vs VSCode (VSIX) when there are multiple editor types per file

[jmenzies12]

Question

Hey there,

We have a VSIX plugin that we deploy into a Theia browser instance. Part of our main functionality is that we have multiple different editor per file, for example a table editor and a DSL (Langium) editor.

Whilst testing some new navigation features we noticed that if you have both editors open, the selection method on the TextEditor becomes flakey. It might work initially but after that it stops highlighting the text until you close both of the editors and reopen. It works fine when you solely have the text editor open. And the issue doesn't happen at all in VSCode.

Currently using Theia 1.68 and I made sure our vscode engine versions matches the supported version for Theia

The function we use for this is fairly simple:

import { Range } from 'vscode-languageserver-types'
import * as vscode from 'vscode'

export async function navigateToDsl(path: string, range?: Range) {
	const uri = vscode.Uri.file(path)
	const document = await vscode.workspace.openTextDocument(uri)
	const editor = await vscode.window.showTextDocument(document)

	const startPosition = new vscode.Position(range?.start?.line ?? 0, range?.start?.character ?? 0)
	const endPosition = new vscode.Position(range?.end?.line ?? 0, range?.end?.character ?? 0)
	editor.selection = new vscode.Selection(startPosition, endPosition)

	editor.revealRange(new vscode.Range(startPosition, endPosition), vscode.TextEditorRevealType.InCenter)
}

Cheers
Jonathan

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[martin-fleck-at]

@jmenzies12 Hi Jonathan! That is very interesting and purely from the code I agree that it should work. Could you let me a little bit more about your setup: You have editors for that file (text and table) and both of them are open at open at the same time. Are they both on the root level? Do they share the same URI? Which one is the default? Does it make a difference which editor was opened first? I tried to test this on a similar project but the command always opens the default editor.

I tried to reproduce this in a project with a similar setup and I can reproduce a behavior where the highlight does not work at all once you open another editor for the same URI. As far as I can tell it is caused by this:

Visibility Change (e.g., selecting another tab) causes the model to be set to null for some workaround:

theia/packages/monaco/src/browser/monaco-editor.ts

Lines 266 to 277 in 793ca84

	handleVisibilityChanged(nowVisible: boolean): void {
	if (nowVisible) {
	this.baseEditor.setModel(this.baseModel);
	this.baseEditor.restoreViewState(this.savedViewState);
	this.baseEditor.focus();
	} else {
	this.savedViewState = this.baseEditor.saveViewState();
	// eslint-disable-next-line no-null/no-null
	this.baseEditor.setModel(null); // workaround for https://github.com/eclipse-theia/theia/issues/14880
	}
	}

Model setting to null leads to editor disposal:

theia/packages/plugin-ext/src/main/browser/text-editor-main.ts

Lines 98 to 100 in 793ca84

	this.toDisposeOnEditor.push(monacoEditor.onDidChangeModel(() => {
	this.setEditor(undefined);
	}));

Which in turn leads to any selection being set to be "ignored" as we do not have the editor anymore:

theia/packages/plugin-ext/src/main/browser/text-editor-main.ts

Lines 142 to 149 in 793ca84

	setSelections(selections: Selection[]): void {
	if (this.editor) {
	this.editor.getControl().setSelections(selections);
	return;
	}
	const monacoSelections = selections.map(TextEditorMain.toMonacoSelections);
	this.setProperties(new TextEditorPropertiesMain(monacoSelections, this.properties!.options, this.properties!.visibleRanges), undefined);
	}

Could you maybe debug your code to confirm that this is the issue? I'm not yet sure why some of the code does this but at we can open an issue for it and maybe you can fix the problem locally if that is the case.

[jmenzies12]

Hey @martin-fleck-at!

Yes, so we have multiple editors that all share the same URI. The standard TextEditor (Langium DSL), the table editor also is based on the CustomTextEditor, lastly we have a diagram editor (which you're more familiar with) using eclipse-glsp (vscode-integration), which I believe is just a CustomEditor (apologies if they are named differently in Theia I'm just going off of the vscode side of things).

The general idea of the functionality is that we have a right click menu in say the glsp diagram, that lets you open the selected element at the related position in the text editor, this is done by sending a language server request to the extension side which then triggers the code snippet I supplied in the original post.

If the standard text editor and either the table or diagram editor are open at the same time the highlighting does not work, occasionally it will work once but then it stops working until both editors are closed.

In this scenario the default editor is the text editor, I did try changing the default editor to the diagram editor and noticed that the command only opens the default editor (which is also inconsistent with vscode api as in vscode it will specifically open the text document, regardless of which document is set as the default).

I tried messing around with opening different editors first and it didn't seem to make a difference.

I'll see if I can debug your suggestions on Theia and get back to you

[jmenzies12]

@martin-fleck-at

Could you maybe debug your code to confirm that this is the issue? I'm not yet sure why some of the code does this but at we can open an issue for it and maybe you can fix the problem locally if that is the case.

Yes, I get the same results as you when debugging.

[martin-fleck-at]

Great, thank you! I opened #17064 for it.

[jmenzies12]

Thanks!

[Pranav-0440]

Hi @martin-fleck-at,

Thanks for opening #17064 👍

I’ve confirmed through debugging that the root cause is indeed the setModel(null) call triggered during visibility changes. Once the model is set to null, the editor instance is effectively disposed on the plugin side, which explains why subsequent editor.selection updates are ignored.

This behavior differs from VS Code, where multiple editors sharing the same URI do not invalidate the text editor instance in this way.

If helpful, I’m happy to:

• Test potential fixes
• Try a local patch
• Help validate behavior against VS Code parity

Thanks again for investigating this so quickly.

[martin-fleck-at]

Hi @Pranav-0440,

I opened a potential fix here: #17065. If you could confirm that this also solves your problem, that would already be a big help, thank you!