[Fix] Font fallback & sidebar page nav font #12948
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Pull Request Overview
This PR updates the parseFont function to always append Streamlit's default font as a final fallback when custom fonts are used. This ensures that if custom fonts fail to load, Streamlit's default fonts are used instead of the browser's defaults.
Key Changes
- Modified
parseFontfunction to accept an optionalfallbackFontparameter and always append it to the font string - Updated all call sites of
parseFontto pass appropriate fallback fonts (sans-serif for body/heading, monospace for code) - Updated tests to reflect new behavior where all custom fonts have Streamlit's default fallback appended
- Added navigation component styling to use theme's bodyFont
Reviewed Changes
Copilot reviewed 3 out of 9 changed files in this pull request and generated 2 comments.
| File | Description |
|---|---|
| frontend/lib/src/theme/utils.ts | Modified parseFont to accept fallback parameter and always append it; updated all parseFont calls to pass appropriate fallbacks |
| frontend/lib/src/theme/utils.test.ts | Updated test expectations to match new behavior; added tests for custom fallback parameter |
| frontend/app/src/components/Navigation/styled-components.ts | Added fontFamily styling to sidebar navigation container |
| e2e_playwright/snapshots/linux/custom_url_base_test/custom_theme_with_url_base[webkit].png | Updated snapshot to reflect visual changes from font fallback behavior |
sfc-gh-bnisco
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Nov 6, 2025
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Describe your changes
This PR resolves 2 font related issues for custom themes:
Sidebar page navigation menu should use sidebar font
Current:
After:
Fallback to Streamlit default font instead of browser default when font file can’t be found
Current: result when
theme.fontset to invalid sourceAfter:
Testing Plan