Keep the flux into the current selection highlighted

atlas.css

@@ -26,9 +26,15 @@
 .flow { stroke:none; transition:opacity .15s; }
 .flow-annotated { fill:#40916f; opacity:.42; }
 .flow-missing { fill:#cfd5c9; opacity:.6; }
+/* The flux feeding the current selection stays emphasised, so the trunk you
+   have walked reads at a glance against the lineages you passed over. */
+.flow.is-path { opacity:.74; }
+.flow-annotated.is-path { fill:#39875f; }
+.flow-missing.is-path { fill:#c2c9ba; }
 /* Lighting a group also lights the flux arriving at it, so you can see where
    a group's assemblies came from. Applied from atlas.js, since the ribbons are
-   painted under every node and cannot be reached with a CSS sibling rule. */
+   painted under every node and cannot be reached with a CSS sibling rule.
+   Listed after the trunk rules so a hover still wins on the selected path. */
 .flow.is-lit { opacity:.9; }
 .flow-annotated.is-lit { fill:#2f7d5c; }
 .flow-missing.is-lit { fill:#b4bdac; }

taxonomy.py

@@ -249,8 +249,11 @@ class Taxonomy:
 
         def ribbon(y0, a0, a1, y1, b0, b1, kind, n):
             bend = (y0 + y1) / 2
+            # The flux into a selected group is the trunk you have walked, so it
+            # stays emphasised rather than waiting for a hover.
+            trunk = " is-path" if n['selected'] else ""
             return (f'<path d="M{a0:.1f},{y0} C{a0:.1f},{bend} {b0:.1f},{bend} {b0:.1f},{y1} '
-                    f'L{b1:.1f},{y1} C{b1:.1f},{bend} {a1:.1f},{bend} {a1:.1f},{y0} Z" class="flow flow-{kind}" '
+                    f'L{b1:.1f},{y1} C{b1:.1f},{bend} {a1:.1f},{bend} {a1:.1f},{y0} Z" class="flow flow-{kind}{trunk}" '
                     f'data-node="{n["uid"]}" data-detail="{escape(n["detail"], quote=True)}"/>')
 
         for depth, column in enumerate(columns):

tests/test_taxonomy.py

@@ -164,6 +164,19 @@ class TaxonomyTests(unittest.TestCase):
             self.assertTrue(path.startswith("2759/"), name)
             self.assertTrue(path.endswith(f"/{taxid}"), name)
 
+    def test_selected_path_flux_stays_highlighted(self):
+        for path in (DEFAULT_PATH, "2759/33154/33208", "2759/33154/33208/6072/33213"):
+            html = self.taxonomy.view(path)
+            chart = html[html.index("<svg"):html.index("</svg>")]
+            trunk = re.findall(r'class="flow flow-(\w+) is-path" data-node="([\d-]+)"', chart)
+            selected = re.findall(r'<g class="tree-node is-selected"[^>]*data-node="([\d-]+)"', chart)
+            # One level transition per step below the root, each carrying the
+            # annotated and not-annotated halves of one group's inflow.
+            self.assertEqual(sorted(set(uid for _, uid in trunk)), sorted(selected[1:]), path)
+            self.assertLessEqual(len(trunk), 2 * (len(selected) - 1), path)
+            # A trunk ribbon is still an ordinary ribbon, hoverable like any other.
+            self.assertNotIn('class="flow flow-annotated is-path" data-node="0-0"', chart)
+
     def test_rejects_paths_outside_the_tree(self):
         for path in ("1", "33208", "2759/33208", "2759/2759~3", "2759/9606", "abc"):
             with self.assertRaises(ValueError, msg=path):