docs: document Accept-header SPA fallback pattern (follow-up to #729)

docs/gotchas.md

@@ -140,6 +140,35 @@ store files. Any subsequent write into the copied tree (adding new
 files in subdirectories) fails with `EPERM`. Fix: pass
 `--no-preserve=mode,ownership` so the output tree is writable.
 
+## SPA fallback: use `Accept` header map, not `try_files ... /index.html`
+
+The naive nginx pattern for a path-prefix SPA (`try_files $uri $uri/
+/matrix/index.html`) silently swallows asset 404s — a missing JS file
+returns `index.html` with a 200, so the JS runtime never loads and the
+page renders blank with no visible error (#685, fixed #684, narrowed #686).
+Extension allowlists (tried in #686 → PR #729) have the same maintenance
+problem: any new file extension the SPA ships breaks silently.
+
+The correct pattern (landed in PR #729, `hive-gateway.nix`) keys the
+fallback on the HTTP `Accept` header:
+
+```nginx
+# Outside the server block (appendHttpConfig):
+map $http_accept $matrix_spa_target {
+  default          "/__matrix_spa_no_html_fallback";
+  "~*text/html"    "/matrix/index.html";
+}
+
+# Inside the location:
+try_files $uri $uri/ $matrix_spa_target =404;
+```
+
+Top-frame navigations always send `Accept: text/html,...` (chrome /
+firefox / safari are consistent). Asset fetches (`image/*`,
+`application/javascript`, `*/*`) don't carry `text/html`, so they
+fall through to the trailing `=404`. No extension list to maintain;
+no named-location indirection needed.
+
 ## `hive-forge`: prefer over raw curl pipelines
 
 Every agent container has `hive-forge` in PATH (installed via