Step through pivot depth instead of listing it

One button per level grows with the pivot: the saved layouts here run 3-5
group_by and 2-3 split_by, so "Customer by Program" rendered ten depth
buttons, and with the layout menu, Save and Fit that is thirteen controls
in the view zone. The active level was a blue button among the greys, so
reading the current state meant scanning for it.

A stepper is three controls whatever the depth and names where you are.
The full list is still there behind the middle button, for jumping rather
than stepping.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -238,10 +238,18 @@ Both axes collapse the same way: a **depth in `ViewConfig`**, set through
 - **Columns** — `split_by_depth`, driven by the `Columns` buttons via
   `applySplitDepth()`
 
-Both sets are drawn by `components/DepthButtons.jsx` from the axis's own field
-list (`groupFull` / `splitFull`), so each button is named after the level it
-reveals. The row axis used to be a fixed `0 1 2 3`, which named nothing and
-offered levels a two-field `group_by` does not have.
+Both axes are drawn by `components/DepthButtons.jsx` from the axis's own field
+list (`groupFull` / `splitFull`), as a **stepper**: `◂ partgroup ▸`, with the
+middle button opening the full level list for a jump rather than a step.
+
+Three controls per axis whatever the depth. It was one button per level, which
+grows with the pivot — a 5-by-3 layout rendered ten of them — and the current
+level was a blue button among the greys, so reading the state meant scanning.
+Before that the row axis was a fixed `0 1 2 3`, which named nothing and offered
+levels a two-field `group_by` does not have.
+
+`depth` is null until the first layout has been read back; the stepper shows an
+em dash rather than guessing a level.
 
 Both are **1-based**: they count the levels to show, where the imperative
 `view.set_depth()` counts the boundary below them. `server.cpp` does

ui/src/components/DepthButtons.jsx

@@ -1,26 +1,79 @@
-// One axis's collapse control. Rows and columns are the same thing — a depth in
-// ViewConfig, counting the levels to show — so they are drawn by one component
-// rather than two that happen to look alike.
+import { useEffect, useRef, useState } from 'react'
+
+// One axis's collapse control, as a stepper.
 //
-// The buttons are named after the level they reveal. The row axis used to be a
-// fixed 0 1 2 3: a number means nothing without already knowing what group_by
-// holds, and a fixed range offers levels that do not exist while hiding ones
-// that do.
+// Rows and columns are the same thing — a depth in ViewConfig, counting the
+// levels to show — so one component draws both.
+//
+// It used to be one button per level, which is N+1 controls that grow with the
+// pivot: a 5-by-3 layout rendered ten of them, and the active level was a blue
+// button somewhere among the greys, so reading the current state meant scanning.
+// A stepper is three controls whatever the depth, and says where you are rather
+// than making you find it. The full list is still one click away, for jumping
+// rather than stepping.
 export default function DepthButtons({ label, levels, depth, onPick, totalLabel = 'Total' }) {
+  const [open, setOpen] = useState(false)
+  const boxRef = useRef(null)
+
+  useEffect(() => {
+    if (!open) return
+    const onDown = (e) => { if (!boxRef.current?.contains(e.target)) setOpen(false) }
+    const onKey  = (e) => { if (e.key === 'Escape') setOpen(false) }
+    // Capture phase: the pivot lives in a shadow root and retargets its events,
+    // so a bubbled listener sees the host rather than the real target.
+    document.addEventListener('mousedown', onDown, true)
+    window.addEventListener('keydown', onKey)
+    return () => {
+      document.removeEventListener('mousedown', onDown, true)
+      window.removeEventListener('keydown', onKey)
+    }
+  }, [open])
+
+  const max = levels.length
+  // depth is null until the first layout has been read back. Showing the last
+  // level would be a guess; an em dash is honest and the arrows still work.
+  const known   = depth != null
+  const current = !known ? '—' : (depth === 0 ? totalLabel : levels[depth - 1])
+  const step    = (d) => onPick(Math.min(max, Math.max(0, d)))
+
+  const arrow = 'border border-gray-200 rounded px-1 py-0.5 text-gray-500 leading-none ' +
+                'hover:border-gray-400 disabled:opacity-30 disabled:hover:border-gray-200 transition-colors'
+
   return (
-    <div className="flex items-center gap-1.5">
-      <span className="pf-section-label">{label}</span>
-      {Array.from({ length: levels.length + 1 }, (_, n) => (
-        <button key={n} onClick={() => onPick(n)}
+    <div ref={boxRef} className="relative flex items-center gap-1">
+      <span className="pf-section-label mr-0.5">{label}</span>
+
+      <button className={arrow} disabled={!known || depth <= 0}
+        title={`Collapse ${label.toLowerCase()} by one level`}
+        onClick={() => step((depth ?? 0) - 1)}>◂</button>
+
+      <button onClick={() => setOpen(o => !o)}
+        title={`${label}: showing ${current}`}
+        className={`border rounded px-2 py-0.5 max-w-[10rem] truncate transition-colors
+          ${open ? 'border-blue-300 text-blue-700 bg-blue-50'
+                 : 'border-gray-200 text-gray-600 hover:border-gray-400'}`}>
+        {current}
+      </button>
+
+      <button className={arrow} disabled={!known || depth >= max}
+        title={`Expand ${label.toLowerCase()} by one level`}
+        onClick={() => step((depth ?? 0) + 1)}>▸</button>
+
+      {open && (
+        <div className="absolute top-full left-0 mt-1 z-50 min-w-[10rem] bg-white border
+                        border-gray-200 rounded shadow-lg py-1 text-xs">
+          {Array.from({ length: max + 1 }, (_, n) => (
+            <button key={n} onClick={() => { onPick(n); setOpen(false) }}
               title={n === 0
                 ? `Collapse ${label.toLowerCase()} to a single total`
                 : `Show ${label.toLowerCase()} down to ${levels.slice(0, n).join(' › ')}`}
-          className={`border rounded px-1.5 py-0.5 transition-colors max-w-[9rem] truncate
-            ${depth === n ? 'border-blue-300 text-blue-600 bg-blue-50'
-                          : 'border-gray-200 text-gray-500 hover:border-gray-400'}`}>
+              className={`block w-full text-left px-3 py-1 truncate
+                ${depth === n ? 'bg-blue-50 text-blue-700' : 'text-gray-700 hover:bg-gray-50'}`}>
               {n === 0 ? totalLabel : levels[n - 1]}
             </button>
           ))}
         </div>
+      )}
+    </div>
   )
 }