Text measurement keeps stale canvas/document context after setEnv() switches document

[antonshevelov]

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• I agree to follow this project's Code of Conduct
• I searched existing issues and discussions

Version

7.2.0

Environment

Chrome, Safari, Firefox

Link To Reproduction

No response

Steps To Reproduce

1. Create an iframe-based preview/editor.
2. Point Fabric to the iframe environment with fabric.setEnv({ ...fabric.getEnv(), window, document }).
3. Create/render a fabric.Textbox and trigger text measurement.
4. Destroy that iframe and create a new one.
5. Call fabric.setEnv() again with the new iframe window/document.
6. Render or measure the same text again.

In our app this reproduces consistently after:

1. opening the editor,
2. viewing banner text,
3. leaving the editor,
4. opening the editor again.

Expected Behavior

After fabric.setEnv() switches to a new document, Fabric should recreate the internal text measuring canvas/context for the current document.

Text metrics should stay correct after iframe recreation or editor reopen.

Actual Behavior

Fabric keeps using the original cached measuring context created for the first document/window.

After the iframe is destroyed and recreated, text measurement still uses the stale context from the old document. This causes incorrect text metrics, and text can visually collapse / "stick together" after reopening the editor.

Additional Context

This looks related to the module-level measuringContext cache used by text measurement.

The context is created once and then reused forever, even after fabric.setEnv() changes the active document. In iframe-based apps this means the measuring context may belong to a destroyed iframe window/document.

A fix like this resolves the issue on our side:

let measuringContext: CanvasRenderingContext2D | null;
let measuringContextDocument: Document | null;

function getMeasuringContext() {
  const fabricDocument = getFabricDocument();

  if (!measuringContext || measuringContextDocument !== fabricDocument) {
    const canvas = createCanvasElementFor({ width: 0, height: 0 });
    measuringContext = canvas.getContext('2d');
    measuringContextDocument = fabricDocument;
  }

  return measuringContext;
}

[asturur]

I don't think we ever planned for the env to be refreshed/created.
if you want start to open a PR to fix this issue and we can take it from there.

Before doing that, either check the code of conduct checkbox or explain what part of the code of conduct is not acceptable to you.