Fix webview font-family so it falls back to monospace for custom editor.fontFamily

[Alexoswin]

Problem

Reported in #324256: when editor.fontFamily is set to a custom font that either doesn't exist or doesn't itself include a generic monospace fallback, code blocks and inline code in the Markdown preview render in the browser's default font (often serif) instead of falling back to a monospace font.

Root cause

Every webview (including the Markdown preview) gets its --vscode-editor-font-family CSS variable from WebviewThemeDataProvider.getWebviewThemeData() in src/vs/workbench/contrib/webview/browser/themeing.ts. That code used the raw editor.fontFamily setting verbatim:

const editorFontFamily = configuration.fontFamily || EDITOR_FONT_DEFAULTS.fontFamily;

extensions/markdown-language-features/media/markdown.css consumes this variable with a var(--vscode-editor-font-family, <fallback list>), but that CSS fallback list only applies when the variable is unset — since themeing.ts always sets it to some string, the CSS-level fallback never kicks in.

The real Monaco text editor doesn't have this problem because BareFontInfo.getMassagedFontFamily() (src/vs/editor/common/config/fontInfo.ts) always appends the platform's default font family (which itself ends in the generic monospace keyword) as a fallback whenever the configured font differs from the default. themeing.ts never used this helper, which is the actual gap.

Fix

• Extracted the fallback-appending logic out of BareFontInfo.getMassagedFontFamily() into a standalone exported function, applyFontFamilyFallback(), in fontInfo.ts (no behavior change for existing callers — getMassagedFontFamily() is now a thin wrapper around it).
• themeing.ts now calls applyFontFamilyFallback() when computing the webview's editorFontFamily, so --vscode-editor-font-family always carries a monospace fallback chain, matching the real editor's behavior.

This fixes the Markdown preview specifically, and also fixes the same issue for any other webview that relies on --vscode-editor-font-family.

Testing performed

• Added src/vs/editor/test/common/config/fontInfo.test.ts covering applyFontFamilyFallback(): custom font gets the fallback appended, a font with spaces gets quoted before the fallback is appended, an already-escaped font family isn't double-wrapped, and a font family equal to the platform default isn't duplicated.
• tsc -p src/tsconfig.json --noEmit passes with no errors.
• eslint passes on all changed files.
• Manually verified the extracted fallback logic against the four test cases above with a standalone script producing the expected output on this platform's default ('Droid Sans Mono', monospace on Linux).

Fixes #324256

[Copilot]

Pull request overview

Aligns webview editor-font fallback behavior with Monaco, preventing serif rendering when custom fonts are unavailable.

Changes:

• Extracts reusable font-family fallback logic.
• Applies it to webview theme data.
• Adds unit coverage for fallback behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/vs/editor/common/config/fontInfo.ts	Extracts applyFontFamilyFallback.
src/vs/editor/test/common/config/fontInfo.test.ts	Tests fallback and quoting behavior.
src/vs/workbench/contrib/webview/browser/themeing.ts	Applies fallback to webview editor fonts.