fix: stop browsers running a cached noVNC against the desktop's own

The desktop failed to start in a browser that had cached noVNC 1.6
from the Debian desktop image: its vnc.html, which has no clipboard
Clear button, ran the Ubuntu image's 1.3 ui.js, which wires one up
and threw "Cannot read properties of null". websockify sends only
Last-Modified, so browsers cache noVNC on a guess, and it answers a
revalidation with 304 when the file on disk is older than the copy.

Serve noVNC through websockify-no-store, which adds Cache-Control:
no-store to every response, and from /novnc/ rather than /, so no
browser holds a stale copy at the paths the desktop now uses.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -4,6 +4,16 @@ Notable changes to kubestronaut-sim, newest first. A version is the `VERSION` st
 (year.month.day.n); `./ksim help` and the top bar of the web page show the one you are running.
 Every entry names the commit it came in.
 
+## Unreleased
+
+### Fixed
+
+- The desktop could fail as it started - "noVNC encountered an error: Cannot read properties of
+  null (reading 'addEventListener')" - in a browser that had cached the Debian desktop's noVNC
+  1.6: its page ran the Ubuntu image's 1.3 `ui.js`. websockify now sends `Cache-Control:
+  no-store`, and noVNC moved to `/novnc/`, past what browsers hold at the old paths. Rebuild the
+  desktop (`./ksim build`) and restart `./ksim web`.
+
 ## 2026.10.19.1 — 2026-10-21
 
 Merged into `main` from `feat/lessons`. Everything below came in `0e5f768`, except the CKS Q103

images/desktop/Containerfile

@@ -29,7 +29,8 @@ RUN useradd -m -s /bin/bash -u 1000 candidate \
  && install -d -o candidate -g candidate /home/candidate/.config/xfce4/xfconf/xfce-perchannel-xml \
  && install -m644 -o candidate -g candidate /etc/xdg/xfce4/panel/default.xml \
       /home/candidate/.config/xfce4/xfconf/xfce-perchannel-xml/xfce4-panel.xml \
- && chmod +x /usr/local/bin/start-desktop
+ && ln -s /usr/share/novnc /srv/novnc \
+ && chmod +x /usr/local/bin/start-desktop /usr/local/bin/websockify-no-store
 USER candidate
 WORKDIR /home/candidate
 EXPOSE 6080

images/desktop/rootfs/usr/local/bin/start-desktop

@@ -9,4 +9,6 @@ Xvnc :1 -geometry "${GEOMETRY:-1600x900}" -depth 24 -rfbport 5901 -localhost \
 for _ in $(seq 50); do [ -S /tmp/.X11-unix/X1 ] && break; sleep 0.1; done
 cd "$HOME"
 dbus-launch --exit-with-session startxfce4 >/tmp/xfce.log 2>&1 &
-exec websockify --web /usr/share/novnc 6080 127.0.0.1:5901
+# noVNC is at /novnc/ (/srv/novnc), not /: browsers may still hold the Debian image's noVNC 1.6 at
+# the old paths, cached before websockify-no-store.
+exec websockify-no-store --web /srv 6080 127.0.0.1:5901

images/desktop/rootfs/usr/local/bin/websockify-no-store

@@ -0,0 +1,21 @@
+#!/usr/bin/python3
+# websockify, with Cache-Control: no-store on every file it serves. On its own it sends only
+# Last-Modified, so a browser keeps noVNC's files for months on a guess, and answers a revalidation
+# with 304 whenever the file on disk is older than the one cached. When the image's noVNC changes
+# (Debian's 1.6 to Ubuntu's 1.3), a cached page then runs the other version's ui.js and fails as it
+# starts. Over loopback, fetching every file each time costs nothing.
+import sys
+
+from websockify import websocketproxy
+
+handler = websocketproxy.ProxyRequestHandler
+end_headers = handler.end_headers
+
+
+def no_store(self):
+    self.send_header("Cache-Control", "no-store")
+    end_headers(self)
+
+
+handler.end_headers = no_store
+sys.exit(websocketproxy.websockify_init())

sim/desktop.py

@@ -28,7 +28,7 @@ DOCS = {
 
 
 def url():
-    return f"http://127.0.0.1:{PORT}/vnc.html?autoconnect=1&resize=remote&reconnect=1&reconnect_delay=1500&show_dot=1"
+    return f"http://127.0.0.1:{PORT}/novnc/vnc.html?autoconnect=1&resize=remote&reconnect=1&reconnect_delay=1500&show_dot=1"
 
 
 def running():