Add fallbacks for system fonts to reduce layout shift.

Signed-off-by: Danila Fedorin <danila.fedorin@gmail.com>

chatgpt-adjust-fallback.py

@@ -0,0 +1,90 @@
+"""
+Generate @font-face settings to help make the fallback font look similar
+to the non-fallback font.
+
+Genererated by ChatGTP 5.2-instant. Not human-modified.
+"""
+
+from fontTools.ttLib import TTFont
+
+USE_TYPO_METRICS = 1 << 7
+
+def get_metrics(path):
+    font = TTFont(path)
+
+    head = font["head"]
+    os2 = font["OS/2"]
+    hhea = font["hhea"]
+
+    use_typo = bool(os2.fsSelection & USE_TYPO_METRICS)
+
+    if use_typo:
+        asc = os2.sTypoAscender
+        desc = os2.sTypoDescender
+        gap = os2.sTypoLineGap
+        source = "OS/2.sTypo*"
+    else:
+        asc = hhea.ascent
+        desc = -hhea.descent
+        gap = hhea.lineGap
+        source = "hhea.*"
+
+    # x-height ALWAYS comes from OS/2
+    if not hasattr(os2, "sxHeight") or os2.sxHeight <= 0:
+        raise ValueError(f"{path} has no usable sxHeight")
+
+    x_height = os2.sxHeight
+
+    print("Source", path, source)
+
+    return {
+        "unitsPerEm": head.unitsPerEm,
+        "ascender": asc,
+        "descender": desc,
+        "lineGap": gap,
+        "xHeight": x_height,
+    }
+
+def compute_overrides(target, fallback):
+    # size-adjust: match x-height
+    size_adjust = (
+        (target["xHeight"] / target["unitsPerEm"]) /
+        (fallback["xHeight"] / fallback["unitsPerEm"])
+    )
+
+    # overrides: force target vertical metrics
+    ascent_override = target["ascender"] / target["unitsPerEm"]
+    descent_override = abs(target["descender"]) / target["unitsPerEm"]
+    line_gap_override = target["lineGap"] / target["unitsPerEm"]
+
+    return {
+        "size_adjust": size_adjust * 100,
+        "ascent_override": ascent_override * 100,
+        "descent_override": descent_override * 100,
+        "line_gap_override": line_gap_override * 100,
+    }
+
+def emit_css(family_name, local_name, values):
+    return f"""
+@font-face {{
+  font-family: "{family_name}";
+  src: local("{local_name}");
+  size-adjust: {values['size_adjust']:.2f}%;
+  ascent-override: {values['ascent_override']:.2f}%;
+  descent-override: {values['descent_override']:.2f}%;
+  line-gap-override: {values['line_gap_override']:.2f}%;
+}}
+""".strip()
+
+# ---- Example usage ----
+
+target = get_metrics("static/fonts/gen/Lora-Regular.woff2")
+fallback = get_metrics("/System/Library/Fonts/Supplemental/Times New Roman.ttf")
+
+values = compute_overrides(target, fallback)
+
+print(emit_css(
+    family_name="Lora Fallback",
+    local_name="Times New Roman",
+    values=values,
+))