New markup over an old stylesheet

Reported from a desktop browser: a stray close button beside the logo, badly
drawn, and a page that would not scroll. None of it was in the code that was
running -- it was the code the browser had not fetched.

The worker caches /static/ under a cache named for the release while the files
in it carried no version, and a page is fetched network-first. That only ever
worked because the worker used to seize every open tab the instant it installed
and wipe the old cache. 1.1.0 stopped it doing that, rightly -- it was swapping
stylesheets out from under a streaming reply -- and a momentary mismatch became
a permanent one: new markup over the previous release's CSS for as long as the
old worker lived. `.sidebar__close` had no rule there, so `.btn--icon` made it
inline-flex: visible everywhere, placed by nothing.

Every /static/ URL carries the release now, written by `templating.asset` and
precached by `sw.js:versioned` -- both halves, because caches.match compares the
query too and precaching the bare path would cache entries nothing requests.
Self-correcting: updating is enough.

The header was also a brand with a button appended and margin-left:auto doing
the placing, which holds exactly while that button is last. Two slots now: a
brand that shrinks and truncates, and a rail on the trailing edge.

Verified before changing anything: with the current stylesheet the button is
display:none at 1280 and, with thirty chats and forty messages, both scrollers
scroll. The first measurement said the thread did not -- that was
scroll-behavior: smooth reporting where it started.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/lembas/web/templating.py

@@ -61,6 +61,34 @@ templates.env.filters["tokens"] = highlight_tokens
 templates.env.globals["tool_label"] = tool_labels.label_for
 templates.env.globals["tool_icon"] = tool_labels.icon_for
 
+def asset(path: str) -> str:
+    """A static asset's URL, with the release stamped into it.
+
+    🚨 This is not cache politeness, it is what stops a release drawing itself
+    from two versions at once.
+
+    The service worker caches `/static/...` under a cache named for the
+    release, and a *page* is fetched network-first while its assets come from
+    that cache. So the moment the worker stops taking over open tabs the
+    instant it installs -- which it must, or it swaps the stylesheets under
+    somebody mid-reply -- the new HTML and the old CSS are served together and
+    the interface is subtly wrong until the worker is replaced. That shipped in
+    1.1.0: a close button intended for a phone drawer appeared, unstyled, on
+    every desktop, because the markup knew about it and the stylesheet did not.
+
+    A version in the URL settles it without anybody having to be careful: the
+    new HTML asks for a URL the old cache has never heard of, so it goes to the
+    network. The two can no longer disagree, whichever worker is in charge.
+
+    Not a hash of the file: `__version__` is the one thing that already moves
+    with every release, and a hash would mean reading every asset on every
+    render or a build step, and there is deliberately no build step here.
+    """
+    return f"/static/{path.lstrip('/')}?v={__version__}"
+
+
+templates.env.globals["asset"] = asset
+
 # A finished reply as the sequence of steps it was. A global for exactly the
 # reason the two above are, and it is why turning the bubble into a sequence
 # needed no change in `pages.py`, `post_message`, `regenerate` or the `done`