Debugger: Preserve variables panel contents when switching editor focus

[arjxn-py]

References

Potential fix for #17829

Code changes

Currently, when switching focus between editors, the debugger’s _clearModel() method clears both the callstack and variables scopes:

jupyterlab/packages/debugger/src/service.ts

Lines 710 to 713 in 11a9a7c

	private _clearModel(): void {
	this._model.callstack.frames = [];
	this._model.variables.scopes = [];
	}

This causes the Variables panel to be emptied whenever focus changes, even if a debugging session is still active. In contrast, the Breakpoints panel correctly persists its state across focus changes.

This PR modifies _clearModel() to only clear the callstack frames, allowing the Variables panel to preserve the last known state until updated by the active session.

User-facing changes

Screen.Recording.2025-08-29.at.15.13.15.mp4

Backwards-incompatible changes

[jupyterlab-probot]

Thanks for making a pull request to jupyterlab!
To try out this branch on binder, follow this link:

[martinRenou]

Shouldn't we do the same for stack frames?

If this is the proper fix, then why is _clearModel there and in which other case is it useful?

[arjxn-py]

Shouldn't we do the same for stack frames?

I am not sure about stack frames but I tried removing that as well as while I tested it I didn't notice anything broken.

If this is the proper fix, then why is _clearModel there and in which other case is it useful?

_clearModel is used in several places in the same file (service.ts):

1. In the session setter, when event.event === 'continued' line 117

2. When DebuggerService Continues the execution of the current thread line 219

3. When retrieving frames to get the latest state of variables line 263

4. When the DebuggerService is started line 463

It seems useful to me but maybe @krassowski or @jtpio could give more insight. (sorry for the ping)

[martinRenou]

It seems that callstack panel isn't cleared when switching focus on notebooks:

Screencast.From.2025-09-02.13-03-16.mp4

Is this code this._model.callstack.frames = []; even called? It should right, so why is the callstack panel not being empty?

[krassowski]

What happens if I click on a file in Sources panel, then set a breakpoint there and then switch between that file and the notebook?

[arjxn-py]

What happens if I click on a file in Sources panel, then set a breakpoint there and then switch between that file and the notebook?

Hi, I tried doing this (behaviour looks normal to me):

Screen.Recording.2025-08-29.at.18.45.04.mp4

[krassowski]

Apologies, I meant "Kernel Sources" like this:

I am not even sure if it currently works the way it should, just highlighting that sometimes the files can be open from debugger itself and it feels like they maybe should not clear these panels?

[arjxn-py]

Thanks for more context @krassowski, I don't know if it works the way we expect it to but here it is -

Screen.Recording.2025-08-29.at.19.43.49.mp4

[arjxn-py]

Hi @krassowski, I'm wondering if the visual regression test failure is actually related to the change because when i inspect the test assets it doesn't look like they are related - would be glad to know what you think. Thanks

[krassowski]

No, the mermaid failures are known and tracked in #17820

[martinRenou]

That looks good to me! Thanks!