A tab bar's edge fade that stays hidden until something is off the edge

The local-attached covers were as wide as the scroll shadows and solid for
only 40% of that, so most of each shadow showed at rest -- invisible on Moria,
a grey sliver at both ends of every tab bar on Shire. The covers are now twice
the shadow's width and solid across all of it.

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

tests/test_ui_js.py

@@ -201,6 +201,24 @@ def test_a_tab_switch_moves_only_the_container_that_scrolls():
     assert "outer.scrollTop" in handler
 
 
+def test_the_tab_bar_edge_fade_is_covered_when_nothing_overflows():
+    """The covers were as wide as the shadows and solid for only 40% of that,
+    so 60% of each shadow showed through with nothing to scroll to. On Shire
+    that was a grey sliver at both ends of every tab bar. A cover has to be
+    solid across the whole shadow. Measured in headless Chromium on
+    /admin/prompts and /settings in both themes before and after.
+    """
+    admin = (ROOT / "web/static/css/admin.css").read_text(encoding="utf-8")
+    start = admin.index("background-attachment: local, local, scroll, scroll")
+    block = admin[admin.rindex(".tabs__bar {", 0, start) : start]
+    cover = re.search(r"linear-gradient\(to right, var\(--bg\) (\d+)%, transparent\)", block)
+    shadow = re.search(r"var\(--scrim\), transparent ([\d.]+)rem", block)
+    sizes = re.search(r"background-size: ([\d.]+)rem 100%, [\d.]+rem 100%, ([\d.]+)rem 100%", block)
+    assert cover and shadow and sizes, "the edge-fade rule changed shape; re-check it by eye"
+    solid = float(sizes.group(1)) * int(cover.group(1)) / 100
+    assert solid >= float(shadow.group(1)) == float(sizes.group(2))
+
+
 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