Fix Firefox selection crash in Monaco: handle shadow DOM + null hit result in _doHitTestWithCaretPositionFromPoint

[aesraethr]

Refs

• [Bug] Firefox 131: Selection issues when Monaco is in shadow dom  monaco-editor#4679 — open since 2024-09-12, label bug + firefox
• Mozilla bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1914596
• CSSOM spec: https://drafts.csswg.org/cssom-view/#dom-document-caretpositionfrompoint

What

Two related defensive changes in _doHitTestWithCaretPositionFromPoint (src/vs/editor/browser/controller/mouseTarget.ts):

1. Pass the host's shadow root to caretPositionFromPoint so Firefox can resolve hits inside a shadow tree (parallels the _doHitTestWithCaretRangeFromPoint sibling at line ~990, which already calls dom.getShadowRoot).
2. Null-check the hit result before dereferencing hitResult.offsetNode — the API legitimately returns null for points that don't fall on an addressable text node (e.g. dragging a multi-line selection past the end of the editor).

Why

Firefox 131 changed document.caretPositionFromPoint so that, when the page uses shadow DOM, the third options argument must include {shadowRoots: [...]} to return the actual hit position. Without it, Firefox returns the shadow host (or null). This crashes the editor's selection logic with:

Uncaught Error: can't access property "offsetNode", i is null
  _doHitTestWithCaretPositionFromPoint@editor.api…
  doHitTest@editor.api…

The error is fired through Monaco's unexpectedErrorHandler, surfaces in the console, and pollutes any error-monitoring tool (Sentry, Datadog RUM) listening on window.onerror. The selection still works — it's a hit-test miss after the fact — but every multi-line drag triggers it.

The bug is not just a shadow-DOM concern: contributors have reported the same offsetNode is null crash on the Monaco playground in Firefox 145+ by simply selecting all text. That suggests Firefox's caretPositionFromPoint can return null even without shadow DOM in some drag scenarios, hence the explicit if (!hitResult) return new UnknownHitTestResult(); guard.

Reproduction

silverwind in microsoft/monaco-editor#4679 (Nov 2025):

text selection causes a JS error `can't access property "offsetNode", hitResult is null` to be thrown in Firefox (currently on v145), and this issue seems to be the root cause. I can reproduce on https://microsoft.github.io/monaco-editor/playground.html?source=v0.55.1#example-creating-the-editor-hello-world by just selecting all text in Monaco

erosman in microsoft/monaco-editor#4679 (Dec 2025):

Uncaught Error: can't access property "offsetNode", i is null — happens when selecting text with mouse in the editor and the mouse moves out of the `<div class="monaco-scrollable-element editor-scrollable vs">`

Fix attribution

The shadow-root passthrough was originally proposed in the issue thread by @nate-bow-db (Oct 2024). The community has been shipping it as a postinstall monkey-patch for over a year. This PR lands the same change in the source plus adds the explicit null-guard for the non-shadow-DOM crash path that surfaced more recently.

Risk / scope

• Scoped to one private static method in mouseTarget.ts.
• The shadow-root branch only fires when dom.getShadowRoot(ctx.viewDomNode) returns non-null — so existing non-shadow-DOM consumers (the standard Monaco bundle) hit the same code path as before.
• The null-guard returns new UnknownHitTestResult(), which is the same fallback the function already returns on its other failure path (line ~1080 today). No new return shape.
• dom.getShadowRoot is already imported and used in the sibling _doHitTestWithCaretRangeFromPoint method, so no new import is needed.

[aesraethr]

@microsoft-github-policy-service agree

[Copilot]

Pull request overview

This PR hardens Monaco editor mouse hit-testing for Firefox in mouseTarget.ts, specifically the Gecko-specific caretPositionFromPoint path used during selection. It aligns the Firefox code path with the existing shadow-DOM-aware caret-range logic and prevents a null dereference when Firefox cannot resolve a caret position.

Changes:

• Pass the containing shadow root to document.caretPositionFromPoint(...) when the editor is hosted inside shadow DOM.
• Return UnknownHitTestResult when caretPositionFromPoint(...) returns null instead of dereferencing offsetNode.
• Add inline comments documenting the Firefox/shadow-DOM behavior motivating the defensive handling.