A reload offer only for a page that is older, and a tab switch that moves one scroller

The update toast fired whenever a service worker was waiting, so after every
release it appeared on pages that were already the release -- including one
fetched with Ctrl+Shift+R. It now compares the waiting worker's release with
the page's own, and so does the reload that follows another tab accepting it.

On Admin -> Prompts, visually hidden labels were positioned against the page
and made the document 6771px tall behind an overflow-hidden root; the tab
handler's scrollIntoView then scrolled that root and lifted the shell 56px.
Every scroll region is now a containing block, and the handler moves only the
container that scrolls.

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

tests/test_ui_js.py

@@ -177,6 +177,30 @@ def test_the_tab_reset_finds_the_container_that_actually_scrolls():
     assert "scrollHeight > " in SOURCE
 
 
+def test_a_tab_switch_moves_only_the_container_that_scrolls():
+    """Switching a tab on /admin/prompts lifted the whole shell 56px, with the
+    topbar gone off the top and a strip of bare background under everything.
+
+    Two halves, and either one alone is enough to bring it back. The prompt
+    cards' `.visually-hidden` labels are `position: absolute`. With no
+    positioned ancestor they were placed against the page and stretched the
+    document to 6771px behind an `overflow: hidden` root. And the handler used
+    `scrollIntoView`, which scrolls every ancestor that can scroll, the root
+    included. Measured in headless Chromium at 1640x930 before and after.
+    """
+    app = (ROOT / "web/static/css/app.css").read_text(encoding="utf-8")
+    start = app.index(".scroll-region,")
+    rule = app[start : app.index("}", start)]
+    assert ".admin-scroll" in rule
+    assert "position: relative" in rule
+
+    code = re.sub(r"/\*.*?\*/", "", SOURCE, flags=re.S)
+    start = code.index('closest(".tabs__bar")')
+    handler = code[start : code.index("})();", start)]
+    assert "scrollIntoView" not in handler
+    assert "outer.scrollTop" in handler
+
+
 def test_the_two_ends_of_the_shell_stay_level():
     """The sidebar footer and the composer sit either side of the same vertical
     edge and are both content-sized, so without a common floor they end at