Description

QuLogic

opened on Sep 25, 2026

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Bug summary

The webagg backend uses the onunload attribute of the image to close the websocket:

matplotlib/lib/matplotlib/backends/web_backend/js/mpl.js

Lines 83 to 85 in c1afaec

	this.imageObj.onunload = function () {
	fig.ws.close();
	};

I cannot find any such event for Images, for its parent HTMLElement interface, or for its parent Element class, or even the Node ABC.

The docs for the unload event say that it is only on the global Window interface, with onunload on the body, frameset, and svg elements. It is also not recommended for use.

Code for reproduction

In the browser console:

var img = new Image();
img.on<TAB>

Actual outcome

There is no onunload in the autocomplete in the browser console.

Expected outcome

We should remove the onunload entirely, but I am uncertain as to whether we should be closing the websocket explicitly somewhere else.

Matplotlib Version

main

Matplotlib Backend

WebAgg

Installation

git checkout

Activity

1. QuLogic

   added

   GUI: webagg

   Maintenance

   on Sep 25, 2026
2. kokhlo commented on Sep 26, 2026

   

   kokhlo

   on Sep 26, 2026

   More actions

   Confirmed — onunload doesn't exist anywhere on the Image prototype chain (HTMLImageElement → HTMLElement → Element → Node), so the assignment at mpl.js:83-85 is a plain expando property that no browser has ever invoked. It's been dead code since it was introduced.

   Re: your uncertainty about closing the websocket explicitly — I don't think it's needed:

   • On page teardown the browser is required to close all open WebSocket connections, and the tornado side already handles that path: WebSocketHandler.on_close → FigureManagerWebAgg.remove_web_socket cleans up the server-side set.
   • For nbagg, the comm is closed by Jupyter's own comm lifecycle (comm.on_close in backend_nbagg.py), independent of this handler.

   So removing the handler outright is behavior-neutral (it never fired) and leaves the real cleanup paths untouched. I'll push a PR removing it shortly.

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