feat(dives): twin sets sit below every single-cylinder group

DECISIONS.md

@@ -6583,8 +6583,8 @@ else reads them.
 
 Every preset is offered in both unit systems; `showIn` and `volumeOptionsFor` do not exist, because
 a named preset serves the traveller handed an unfamiliar tank, whom a units filter hides it from.
-`volumeGroupsFor` keeps the rows browsable under four headings (Metric singles, Twin sets, US
-aluminium, US steel), the reader's own two first; headings are `aria-hidden` labels on a
+`volumeGroupsFor` keeps the rows browsable under four headings (Metric singles, US aluminium, US
+steel, Twin sets), the reader's own singles first; headings are `aria-hidden` labels on a
 `role="group"` wrapper with no index, so `nextActiveIndex` skips them.
 
 HP117 and LP95 are both 15.0 L, so the list is keyed by label. The mixture must not record working

src/components/dives/volume-combobox.render.test.tsx

@@ -139,17 +139,17 @@ describe("VolumeCombobox", () => {
   // the groups come in. They are written out rather than derived from
   // `VOLUME_GROUPS` so that a preset filed in the wrong group fails here instead
   // of agreeing with itself.
-  it("offers every preset in metric, with the metric groups first", async () => {
+  it("offers every preset in metric, with the metric singles first", async () => {
     const user = userEvent.setup();
     render(<Harness />);
 
     await user.click(field());
 
     expect(groupNames()).toEqual([
       "Metric singles",
-      "Twin sets",
       "US aluminium",
       "US steel",
+      "Twin sets",
     ]);
     expect(optionNames()).toEqual([
       "3 L",
@@ -160,10 +160,6 @@ describe("VolumeCombobox", () => {
       "15 L",
       "18 L",
       "20 L",
-      "14 L (2x7 L)",
-      "22.2 L (2x AL80)",
-      "24 L (2x12 L)",
-      "30 L (2x15 L)",
       "5.7 L (AL40)",
       "7.1 L (AL50)",
       "9 L (AL63)",
@@ -179,11 +175,17 @@ describe("VolumeCombobox", () => {
       "15 L (LP95)",
       "17 L (LP108)",
       "19 L (LP121)",
+      "14 L (2x7 L)",
+      "22.2 L (2x AL80)",
+      "24 L (2x12 L)",
+      "30 L (2x15 L)",
     ]);
   });
 
   it("offers every preset in imperial, with the US groups first", async () => {
-    // Same set, different order, each US row led by its name. The two 15 L steels
+    // Same set, each US row led by its name, and the twin sets last in both
+    // systems - they belong to neither family, so neither reader's own groups
+    // sit under them. The two 15 L steels
     // are the case a value-keyed list could not render: the HP117 and the LP95
     // differ by a working pressure the mixture does not record, so they share a
     // litre figure and nothing else.
@@ -261,8 +263,8 @@ describe("VolumeCombobox", () => {
 
   it("steps the arrow keys over a group heading rather than onto it", async () => {
     // Headings are not options and hold no index, so the ninth press has to clear
-    // the eight Metric singles and land on the first Twin set. A heading that took
-    // an index of its own would leave the highlight one row short.
+    // the eight Metric singles and land on the first US aluminium row. A heading
+    // that took an index of its own would leave the highlight one row short.
     const user = userEvent.setup();
     render(<Harness />);
 
@@ -272,7 +274,7 @@ describe("VolumeCombobox", () => {
     }
 
     expect(screen.getByRole("option", { selected: true })).toHaveTextContent(
-      "14 L (2x7 L)",
+      "5.7 L (AL40)",
     );
   });
 });

src/components/dives/volume-combobox.tsx

@@ -29,12 +29,13 @@ export interface VolumeOptionGroup {
    * Whose divers this group is *for*, which decides only where it sits.
    *
    * Never whether it is shown: every group is offered to every diver, and
-   * `volumeGroupsFor` moves the reader's own two to the front rather than
-   * dropping the other two. See DECISIONS.md - a list that hides the unfamiliar
-   * half is a list that fails the traveller, who is the one person a named
-   * preset is for.
+   * `volumeGroupsFor` moves the reader's own to the front rather than dropping
+   * the rest. See DECISIONS.md - a list that hides the unfamiliar half is a list
+   * that fails the traveller, who is the one person a named preset is for.
+   *
+   * Absent where a group belongs to neither family, which sinks it below both.
    */
-  system: UnitSystem;
+  system?: UnitSystem;
   options: VolumeOption[];
 }
 
@@ -75,10 +76,11 @@ export const VOLUME_GROUPS: VolumeOptionGroup[] = [
   },
   {
     label: "Twin sets",
-    // Filed metric because three of the four are metric compositions, and a twin
-    // set is asked for by its total either way. The 2x AL80 rides along rather
-    // than splitting the group by material.
-    system: "metric",
+    // No `system`, so this sits under both families whichever way the diver
+    // reads: a twin set is asked for by its total either way, and it is the
+    // rarer pick, so the singles a diver reaches for most are the rows nearest
+    // the box. The 2x AL80 rides along rather than splitting the group by
+    // material.
     options: [
       { value: 14, label: "14 L (2x7 L)" },
       { value: 22.2, label: "22.2 L (2x AL80)", imperialName: "2x AL80" },
@@ -138,21 +140,24 @@ export const VOLUME_GROUPS: VolumeOptionGroup[] = [
 /**
  * The groups in the order a diver reading in `units` sees them.
  *
- * The reader's own two lead and the other two follow; nothing is dropped. That
- * is the whole of the difference, and it is deliberate that membership is not
- * part of it. A preset list filtered by unit system reads sensible and fails the
- * one diver it exists for: hand a metric diver an AL80 on a boat in Florida and
- * the very preset that tells them it holds 11.1 L is the one that has been
- * hidden, and an imperial diver handed a 12 L in Croatia is stuck the same way.
- * The unfamiliar half is the half worth showing - see DECISIONS.md.
+ * The reader's own singles lead, the other family's follow, and the groups
+ * belonging to neither sit at the bottom; nothing is dropped. That is the whole
+ * of the difference, and it is deliberate that membership is not part of it. A
+ * preset list filtered by unit system reads sensible and fails the one diver it
+ * exists for: hand a metric diver an AL80 on a boat in Florida and the very
+ * preset that tells them it holds 11.1 L is the one that has been hidden, and an
+ * imperial diver handed a 12 L in Croatia is stuck the same way. The unfamiliar
+ * half is the half worth showing - see DECISIONS.md.
  *
  * Grouping is what keeps the full list browsable, which is the job the split was
  * wrongly doing.
  */
 export function volumeGroupsFor(units: UnitSystem): VolumeOptionGroup[] {
+  const families = VOLUME_GROUPS.filter((group) => group.system !== undefined);
   return [
-    ...VOLUME_GROUPS.filter((group) => group.system === units),
-    ...VOLUME_GROUPS.filter((group) => group.system !== units),
+    ...families.filter((group) => group.system === units),
+    ...families.filter((group) => group.system !== units),
+    ...VOLUME_GROUPS.filter((group) => group.system === undefined),
   ];
 }