diff --git a/docs/TROUBLESHOOTING.md b/docs/TROUBLESHOOTING.md
index 7d8b3f4..fbde23d 100644
--- a/docs/TROUBLESHOOTING.md
+++ b/docs/TROUBLESHOOTING.md
@@ -161,6 +161,12 @@ LaunchServices dereferenced a nullable error-output pointer in six paths. The pa
 
 LaunchServices now opens `http`, `https` and `mailto` links on the Linux desktop itself. It loads the host's `libsystemd.so.0` and asks `xdg-desktop-portal` (`org.freedesktop.portal.OpenURI`) on the user session bus to open the link, so your default browser starts outside Darling and survives `darling shutdown`. Without a portal, it starts `xdg-open` in a transient systemd user unit instead. It also reports a handler for these schemes, since Electron checks for one before opening a link. Other schemes, control characters and URLs over 4096 bytes are rejected. If a link still does nothing, check the client's stderr for a `LaunchServices:` line and that `busctl --user status org.freedesktop.portal.Desktop` works on the host. Both LaunchServices and AppKit must be rebuilt. The hand-off passed the host-side D-Bus regression and opened the host browser from a guest through both `open(1)` and `NSWorkspace`; a link clicked in the Riot client is not yet confirmed. The null-pointer fix does not explain every unrelated button crash.
 
+## Riot client shows "Not Found" instead of its UI
+
+The client window opens but only shows `Not Found`, and its log has `ENOENT, dist/renderer/index.html not found in …/app.asar` although the archive contains that file. Electron cannot hand out a descriptor into `app.asar`, so its asynchronous `fs.open` copies the file to a temporary file first (`copyFileOut`) and reports a failed copy as "not found". Chromium on macOS takes its temporary directory from `NSTemporaryDirectory()`, not `TMPDIR`, and that directory is unusable here, so every copy fails. Synchronous reads such as `fs.readFileSync` do not copy and keep working, which is why the rest of the client starts.
+
+The launcher sets `MAC_CHROMIUM_TMPDIR=/private/tmp`, which Chromium checks first; `scripts/install-runtime.sh` already creates that guest directory with mode `1777`. To check the archive outside the client, run the client binary as Node (`ELECTRON_RUN_AS_NODE=1`) with a script that calls `fs.createReadStream` on a file inside `app.asar`: it fails without the variable and reads the file with it.
+
 ## Language dropdown closes Riot, or client resize leaves a clipped UI
 
 The [report in issue #1](https://github.com/lucannez64/darling-lol/issues/1#issuecomment-6194498757) contains an Electron renderer abort with `Symbol not found: _CTFontDescriptorCreateMatchingFontDescriptors`. CoreText now exports font-descriptor matching and localized-attribute lookup, and enumerates installed font families through fontconfig instead of returning a null stub. Localized lookup currently falls back to canonical font names. The standalone font-list check exercises Chromium's sequence, including mandatory family matching and retained results.
diff --git a/tools/run-riot-client.sh b/tools/run-riot-client.sh
index 75b6ec7..21fc89f 100755
--- a/tools/run-riot-client.sh
+++ b/tools/run-riot-client.sh
@@ -98,7 +98,13 @@ export __GL_YIELD="${__GL_YIELD:-NOTHING}"
 if [ -w /proc/self/timerslack_ns ]; then
   echo 1 > /proc/self/timerslack_ns 2>/dev/null || true
 fi
+# Chromium takes its temp directory from NSTemporaryDirectory(), which is unusable
+# under Darling, so every temp file fails. Electron opens files in app.asar
+# asynchronously by copying them to a temp file and reports the failed copy as
+# "not found": the Riot client then shows a "Not Found" page instead of its UI.
+# MAC_CHROMIUM_TMPDIR overrides the directory.
 exec env -u ELECTRON_RUN_AS_NODE -u DYLD_INSERT_LIBRARIES EGL_PLATFORM=x11 \
+  MAC_CHROMIUM_TMPDIR="${MAC_CHROMIUM_TMPDIR:-/private/tmp}" \
   LANG="${RCLOL_GUEST_LOCALE:-en_US.UTF-8}" LC_ALL="${RCLOL_GUEST_LOCALE:-en_US.UTF-8}" \
   mesa_glthread="${RCLOL_MESA_GLTHREAD-${mesa_glthread-false}}" \
   XMODIFIERS="${RCLOL_XMODIFIERS:-@im=none}" DISPLAY="${DISPLAY:-:1}" \