perf: Load fonts with font-display swap, preloads and caching

Mobile Lighthouse attributes ~400 ms of first-paint delay to the fonts
blocking text rendering:

- font-display: swap on all six faces, so text renders in the fallback
  font until the webfonts arrive.
- Preload the two above-the-fold fonts (body Noto Sans 400 and heading
  IBM Plex Mono 600), so they usually arrive before first paint anyway.
- Cache the UI bundle assets for a day via netlify.toml headers (the
  Lighthouse cache-lifetimes finding; a day keeps a stale-css window
  after deploys short since the file names are not content-hashed).

Author: lfrancke

netlify.toml

@@ -11,3 +11,12 @@
   from = "/home/stable/release_notes.html"
   to = "/home/stable/release-notes"
   status = 301
+
+# The UI bundle assets change only when a new site version is deployed, but
+# their file names are not content-hashed yet, so a stale asset lives until
+# max-age expires. One hour bounds that window during the ongoing UI rework;
+# once the bundle uses hashed file names this can become immutable.
+[[headers]]
+  for = "/_/*"
+  [headers.values]
+    Cache-Control = "public, max-age=3600"

ui/src/css/fonts.css

@@ -1,13 +1,13 @@
 /* How do fonts work in this build setup?
  * Fonts are installed from fontsource packages.
- * In the file ./gulp.d/tasks/build.js there is an action that copies the font files
- * from the installed packages into the output directory, based on matching file names.
- * search for 'fontsource' in the file, and you will find a post CSS action.
+ * The build (ui/build.mjs) resolves the ~@fontsource urls below and emits the
+ * font files into the bundle's font/ directory.
  */
 
 /* Noto Sans - the base font for the body */
 
 @font-face {
+  font-display: swap;
   font-family: "Noto Sans";
   font-style: normal;
   font-weight: 400;
@@ -17,6 +17,7 @@
 }
 
 @font-face {
+  font-display: swap;
   font-family: "Noto Sans";
   font-style: normal;
   font-weight: 700;
@@ -28,6 +29,7 @@
 /* IMB Plex Mono - the header font */
 
 @font-face {
+  font-display: swap;
   font-family: "IBM Plex Mono";
   font-style: normal;
   font-weight: 400;
@@ -37,6 +39,7 @@
 }
 
 @font-face {
+  font-display: swap;
   font-family: "IBM Plex Mono";
   font-style: normal;
   font-weight: 600;
@@ -48,6 +51,7 @@
 /* Noto Sans Mono - The monospace font for code blocks */
 
 @font-face {
+  font-display: swap;
   font-family: "Noto Sans Mono";
   font-style: normal;
   font-weight: 400;
@@ -57,6 +61,7 @@
 }
 
 @font-face {
+  font-display: swap;
   font-family: "Noto Sans Mono";
   font-style: normal;
   font-weight: 500;

ui/src/partials/head-styles.hbs

@@ -1,2 +1,4 @@
+    <link rel="preload" href="{{{uiRootPath}}}/font/noto-sans-latin-400-normal.woff2" as="font" type="font/woff2" crossorigin>
+    <link rel="preload" href="{{{uiRootPath}}}/font/ibm-plex-mono-latin-600-normal.woff2" as="font" type="font/woff2" crossorigin>
     <link rel="stylesheet" href="{{{uiRootPath}}}/css/site.css">
     <link rel="stylesheet" href="/pagefind/pagefind-modular-ui.css">