Read a null on the column axis as null, not as the text "null"

The engine renders a null split_by value as the literal "null" in the
column name (t_tscalar::to_string on an invalid scalar) and an empty
string as ''. cellFilters() passed "null" through as a value, so a
slice over a blank segment_new became segment_new = 'null', matched no
row, and the adjustment wrote nothing. '' was refused outright, on the
mistaken belief that the two blanks joined to the same text.

Both are now mapped back: "null" to null, '' to ''.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -697,12 +697,16 @@ The panel prints `(null)` and `(blank)` rather than nothing, and `—` only when
 the column is genuinely absent from the slice: a slice isolated to the blank node
 and one the column plays no part in are opposite statements and looked identical.
 
-**A blank on the column axis cannot be isolated.** A column name is its values
+**A blank on the column axis is read back from the column name.** A column
-joined with `|`, and a null joins as the empty string, so by the time the name
+name is its values joined with `|`, each rendered by `t_tscalar::to_string` in
-reaches the client the two blanks are the same text. Those cells are refused —
+`scalar.cpp`, which gives `''` for an empty string and the literal text `"null"`
-with a message, and counted once for a drag — rather than widened, since a slice
+for a null. So `cellFilters()` maps `"null"` to null and `''` to `''`, and both
-missing a split_by level means every value of it. Filtering on the column
+blanks are isolatable on either axis. The one ambiguity left is a dimension
-instead (`scope`, which carries `is null`) is the way to reach them.
+whose value is really the text `null`; none exists in this data.
+
+This used to be believed impossible — the claim was that a null joined as `''`
+too — so `''` cells were refused, and `"null"` was taken at its word: the slice
+became `segment_new = 'null'`, matched no row, and the adjustment reported zero.
 
 **The pivot's own filter travels alongside, as `scope`.** A slice is
 `{col: value}` and can only mean equality, so a view narrowed to

ui/src/views/Forecast.jsx

@@ -369,7 +369,7 @@ export default function Forecast({ sources = [], sourceId, versions = [], versio
   // attaches to a click. Row dimensions come from the view's __ROW_PATH__ (raw values,
   // so dates stay epoch millis rather than whatever the grid formatted them as), and
   // column dimensions from the split_by segments of the column name.
-  async function slicesFromRegion(win, { quiet = false } = {}) {
+  async function slicesFromRegion(win) {
     const viewer = viewerRef.current
     if (!viewer) return []
     const cfg  = await viewer.save()
@@ -382,10 +382,6 @@ export default function Forecast({ sources = [], sourceId, versions = [], versio
     const c0 = win.start_col ?? 0
     const c1 = win.end_col ?? userKeys.length
     const out = []
-    // cells whose column-axis value is blank, which cannot be isolated (see
-    // cellFilters) -- counted so the drag can say why it skipped them once,
-    // rather than per cell
-    let blankCols = 0
     for (let i = 0; i < rows.length; i++) {
       const path = rows[i].__ROW_PATH__ || []
       // the grand-total row resolves to no dimension at all, which would mean
@@ -394,8 +390,7 @@ export default function Forecast({ sources = [], sourceId, versions = [], versio
       for (let c = c0; c < c1; c++) {
         const key = userKeys[c]
         if (!key) continue
-        const { rowFilters, colFilters, colBlank } = cellFilters(cfg, rows[i], key)
+        const { rowFilters, colFilters } = cellFilters(cfg, rows[i], key)
-        if (colBlank) { blankCols++; continue }
         if (groupBy.length && !rowFilters.length) continue
         const slice = sliceFromFilters([...base, ...rowFilters, ...colFilters],
                                        (cfg.columns || []).filter(Boolean))
@@ -404,11 +399,6 @@ export default function Forecast({ sources = [], sourceId, versions = [], versio
         out.push({ slice, area: { x0: c, x1: c, y0: y, y1: y } })
       }
     }
-    // Quiet for a relocate, which sweeps the whole grid rather than a region the
-    // user dragged: the blanks it passes over are not cells anyone asked for.
-    if (blankCols && !quiet) {
-      flash(`${blankCols} cell${blankCols > 1 ? 's' : ''} skipped: a blank column-axis value cannot be isolated`, 'error')
-    }
     return out
   }
 
@@ -462,7 +452,7 @@ export default function Forecast({ sources = [], sourceId, versions = [], versio
       // any ordinary depth it is a handful. Fully expanded it is the whole
       // traversal, and a slow redraw of a highlight is not worth having.
       if (!n || n > MAX_RELOCATE_ROWS) { areasRef.current.clear(); paintAreas(); return }
-      const found = await slicesFromRegion({ start_row: 0, end_row: n }, { quiet: true })
+      const found = await slicesFromRegion({ start_row: 0, end_row: n })
       const live  = new Set(slicesRef.current.map(sliceKey))
       const next  = new Map()
       for (const { slice, area } of found) {
@@ -1101,11 +1091,7 @@ export default function Forecast({ sources = [], sourceId, versions = [], versio
         const measures = (config.columns || []).filter(Boolean)
         let s
         if (cell) {
-          const { rowFilters, colFilters, colBlank } = cellFilters(config, detail.row, colKey)
+          const { rowFilters, colFilters } = cellFilters(config, detail.row, colKey)
-          if (colBlank) {
-            flash('A blank column-axis value cannot be isolated — filter on it instead', 'error')
-            return
-          }
           if (!rowFilters.length) return
           s = sliceFromFilters([...(config.filter || []), ...rowFilters, ...colFilters], measures)
         } else {
@@ -2590,11 +2576,18 @@ function sliceFromFilters(filters, measures = []) {
 // a dimension whose value is 0 goes the same way. The row path is the raw truth
 // and carries the blank, so the slice is built from that instead.
 //
-// A blank on the *column* axis cannot be recovered at all: a column name is its
+// A blank on the *column* axis arrives as text, and the two blanks arrive as
-// values joined with '|', and a null joins as the empty string, so the two are
+// different text: a column name is its values joined with '|', each rendered by
-// the same text by the time it reaches us. Those cells are refused rather than
+// t_tscalar::to_string, which gives the empty string for '' and the literal
-// widened -- `colBlank` says so -- since a slice missing a split_by level means
+// "null" for a null (scalar.cpp -- an invalid scalar returns {"null"} before the
-// every value of it.
+// type switch). So "null" is read back as null and '' as ''. Both used to be
+// mishandled: '' was refused as unrecoverable, on the belief that a null joined
+// as '' too, and "null" was taken at its word -- a slice of segment_new =
+// 'null', which matches no row, so the adjustment wrote nothing.
+//
+// The one thing this cannot tell apart is a null from a dimension value that is
+// really the four-letter text 'null'. None exists in this data; one would be
+// read as null.
 function cellFilters(cfg, row, colKey) {
   const groupBy = cfg.group_by || []
   const splitBy = cfg.split_by || []
@@ -2603,7 +2596,6 @@ function cellFilters(cfg, row, colKey) {
     .map((col, ix) => (ix < path.length && path[ix] !== undefined ? [col, '==', path[ix]] : null))
     .filter(Boolean)
   let colFilters = []
-  let colBlank   = false
   if (colKey != null) {
     // A column name is dimension values joined by | with the measure last, and
     // only as many values as the axis currently shows -- collapse the column
@@ -2621,12 +2613,11 @@ function cellFilters(cfg, row, colKey) {
         if (v === undefined) return null
         // the measure name read positionally as a dimension value
         if (META_COL_RE.test(v)) return null
-        if (v === '') { colBlank = true; return null }
+        return [col, '==', v === 'null' ? null : v]
-        return [col, '==', v]
       })
       .filter(Boolean)
   }
-  return { rowFilters, colFilters, colBlank }
+  return { rowFilters, colFilters }
 }
 
 // the user keys of a to_json row, in the order the grid lays its columns out