fix: fetcher.submit failing with plain objects containing tagName pro…

[joseph0926]

Fixes #14526

Summary

Updates isHtmlElement() to use instanceof HTMLElement instead of checking for the presence of a tagName string property. This prevents plain data objects with a tagName property from being incorrectly identified as HTML elements.

The previous implementation would throw an error when submitting objects like { tagName: "div", data: "test" } via fetcher.submit(), as they were mistakenly treated as invalid HTML elements.

Changes

• Updated isHtmlElement() in packages/react-router/lib/dom/dom.ts to use instanceof HTMLElement

• I largely followed the approach described in rossipedia’s comment, and additionally only introduced the following tests and edge-case handling,

  • Added a guard for environments where HTMLElement is not defined (e.g., Node.js)
  • Added test cases to verify that plain objects with a tagName property are handled correctly

Test

• Added new test file packages/react-router/__tests__/dom/fetcher-submit-tagname-test.tsx

[changeset-bot]

🦋 Changeset detected

Latest commit: 5b60a1c

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 11 packages

Name	Type
react-router	Patch
@react-router/architect	Patch
@react-router/cloudflare	Patch
@react-router/dev	Patch
react-router-dom	Patch
@react-router/express	Patch
@react-router/node	Patch
@react-router/serve	Patch
@react-router/fs-routes	Patch
@react-router/remix-routes-option-adapter	Patch
create-react-router	Patch

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[brophdawg11]

Thanks! Would you mind adding a changeset via pnpm changeset?

[joseph0926]

Thanks! Would you mind adding a changeset via pnpm changeset?

58f17cc

I added the changeset

[brophdawg11]

Thanks!