When a canvas belongs to a detached document (e.g. one created via
document.implementation.createHTMLDocument()), document->window()
returns null, causing a null pointer crash in set_font.
Use Length::ResolutionContext::for_document() instead of for_window(),
which handles the no-navigable case gracefully and is already the
recommended pattern (per existing FIXME in Length.h). This also fixes
the same crash path via fillText, strokeText, and measureText which
trigger lazy font initialization through set_font.
Fixes#8515.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
