Fallback font substitution ignored in Inkscape svg-export module

Answer

Inkscape will ignore the fallback font set in Preferences → Fonts when any of the following conditions are true:

1. The requested font is not truly missing; fontconfig returns a similar font (different weight/width) and Inkscape uses that instead of consulting the fallback list.
2. The build is GTK4‑based; GTK4’s internal text rendering can select a system substitute before Inkscape’s fallback preference is checked.
3. The font cache at ~/.config/inkscape/fontcache is stale or corrupted, causing Inkscape to think the font is present or to skip loading the fallback entry.

Confirmed facts (from the research brief)

• Inkscape applies the user‑defined fallback only when fontconfig reports the font as unavailable.
• GTK3 builds rely on Pango, which consults Inkscape’s fallback list after fontconfig fails.
• GTK4 builds use GTK4’s internal text rendering that may return a system substitute before Inkscape’s preference is checked.
• The font cache stores scanned font metadata; clearing it forces a rescan and often restores correct fallback behavior.

Likely explanation

The divergence between GTK3 and GTK4 code paths creates two separate decision points for font selection. In GTK3, after fontconfig fails, Pango hands control back to Inkscape, which then reads the fallback list from Preferences → Fonts. In GTK4, the GTK4 text layer may resolve a substitute font internally and never invoke Inkscape’s fallback logic, so the user setting is bypassed. A corrupted font cache can mislead fontconfig into reporting a font as available, preventing the fallback path from being taken in either build.

Steps to verify and restore correct fallback behavior

1. Create a simple SVG that uses a non‑existent font family, e.g. <text font-family='NonexistentFont'>Test</text>.
2. In Inkscape, open Edit → Preferences → Fonts and set a specific fallback font (e.g., DejaVu Sans).
3. Export the drawing as plain SVG (File → Save As → Plain SVG).
4. Open the exported SVG in a text editor and check the font-family attribute; it should contain the fallback font if the mechanism works.
5. Run Inkscape with verbose logging: inkscape --verbose yourfile.svg and look for console lines like Font not found followed by Using fallback font.
6. Repeat the test with both a GTK3 build (e.g., the official AppImage) and a GTK4 build (e.g., a nightly or distribution package) and compare whether the fallback appears.
7. If the fallback is ignored, close Inkscape, delete or rename the font cache directory: mv ~/.config/inkscape/fontcache ~/.config/inkscape/fontcache.bak, then restart Inkscape and repeat the export test.

Missing diagnostic detail

To give a definitive recommendation, please confirm which GTK variant of Inkscape you are running (GTK3 or GTK4). Knowing the build will determine whether clearing the font cache alone is sufficient or whether a GTK4‑specific workaround is needed.