Duplicate a line even when 10 source bars would pass the horizon.

Prod never saw POST /api/lines: the browser aborted the shift. Clamp to
the last owned source slot so Duplicate still creates a copy.

static/app.js

@@ -1095,6 +1095,14 @@ createApp({
       return chartApi?.shiftLineTime(line, time, count) ?? null;
     }
 
+    function duplicateShiftCount(line, count = 10) {
+      const times = [line.anchor_t, line.last_t];
+      if (line.cutoff_t != null) times.push(line.cutoff_t);
+      const allowed = times.map(time => chartApi?.maxLineShift(line, time, count));
+      if (allowed.some(value => value == null)) return null;
+      return Math.min(...allowed);
+    }
+
     function duplicateName(line) {
       const match = (line.label || `${line.tf} ${line.side}`).match(/^(.*?)(?: (\d+))?$/);
       const base = match[1];
@@ -1105,15 +1113,35 @@ createApp({
     }
 
     async function duplicateLine(line) {
+      const fail = reason => {
+        addEvent(`DUPLICATE #${line.number} FAILED · ${reason}`, 'error');
+        fetch('/api/debug/snap', {
+          method: 'POST',
+          headers: { 'Content-Type': 'application/json' },
+          body: JSON.stringify({
+            kind: 'duplicate',
+            tf: line.tf,
+            note: `#${line.number} ${reason} last_t ${line.last_t} cutoff ${line.cutoff_t}`,
+          }),
+        }).catch(() => {});
+      };
       if (line.geometry_resolved === false) {
-        addEvent(`DUPLICATE #${line.number} FAILED · no source geometry`, 'error');
+        fail('no source geometry');
         return;
       }
-      const anchorT = shiftTimeByBars(line, line.anchor_t);
-      const endT = shiftTimeByBars(line, line.last_t);
-      const cutoffT = line.cutoff_t == null ? null : shiftTimeByBars(line, line.cutoff_t);
+      const count = duplicateShiftCount(line);
+      if (count == null) {
+        fail('cannot shift');
+        return;
+      }
+      const shift = (time, bars) => (count > 0
+        ? shiftTimeByBars(line, time, bars)
+        : chartApi.shiftDisplayedTime(time, 10));
+      const anchorT = shift(line.anchor_t, count);
+      const endT = shift(line.last_t, count);
+      const cutoffT = line.cutoff_t == null ? null : shift(line.cutoff_t, count);
       if (anchorT == null || endT == null || (line.cutoff_t != null && cutoffT == null)) {
-        addEvent(`DUPLICATE #${line.number} FAILED · cannot shift 10 bars`, 'error');
+        fail('cannot shift');
         return;
       }
       const endP = line.anchor_p + line.slope * (line.last_t - line.anchor_t);

static/chart.js

@@ -2585,11 +2585,40 @@ class ConfluenceChart {
     return level.anchor_p + (endPrice - level.anchor_p) * (target - start) / (end - start);
   }
 
+  sourceIndexAt(level, time) {
+    const source = this.sourceSeries(level);
+    if (!source) return null;
+    const exact = ConfluenceChart.timeframeIndexAt(source, time);
+    if (exact != null) return exact;
+    const times = ConfluenceChart.sourceTimes(source);
+    if (!times.length) return null;
+    let lo = 0;
+    let hi = times.length;
+    while (lo < hi) {
+      const mid = (lo + hi) >> 1;
+      if (times[mid] <= time) lo = mid + 1;
+      else hi = mid;
+    }
+    return lo === 0 ? null : lo - 1;
+  }
+
   shiftLineTime(level, time, bars) {
     const source = this.sourceSeries(level);
     if (!source) return this.shiftDisplayedTime(time, bars);
-    const index = ConfluenceChart.timeframeIndexAt(source, time);
-    return index == null ? null : ConfluenceChart.timeframeTimeAt(source, index + bars);
+    const index = this.sourceIndexAt(level, time);
+    if (index == null) return null;
+    const times = ConfluenceChart.sourceTimes(source);
+    const target = Math.min(index + bars, times.length - 1);
+    if (target < 0) return times[0] ?? null;
+    return ConfluenceChart.timeframeTimeAt(source, target);
+  }
+
+  maxLineShift(level, time, bars) {
+    const index = this.sourceIndexAt(level, time);
+    if (index == null) return null;
+    const source = this.sourceSeries(level);
+    const last = ConfluenceChart.sourceTimes(source).length - 1;
+    return Math.max(0, Math.min(bars, last - index));
   }
 
   shiftDisplayedTime(time, slots) {

tests/e2e/trendline.test.mjs

@@ -234,6 +234,8 @@ test('daily future slots and live duration follow the server session calendar',
           beyondHorizon: ConfluenceChart.timeframeTimeAt(source, source.allTimes.length + 1),
           shifted: c.shiftDisplayedTime(thursday, 1),
           shiftBeyondHorizon: c.shiftLineTime(horizonLine, monday, 1),
+          horizonShift: c.maxLineShift(horizonLine, monday, 10),
+          thursdayShift: c.maxLineShift(horizonLine, thursday, 10),
         };
       });
 
@@ -241,7 +243,9 @@ test('daily future slots and live duration follow the server session calendar',
       assert.equal(result.duration, 23 * 3600);
       assert.equal(result.beyondHorizon, null);
       assert.equal(result.shifted, 1786917600);
-      assert.equal(result.shiftBeyondHorizon, null);
+      assert.equal(result.shiftBeyondHorizon, 1787004000);
+      assert.equal(result.horizonShift, 0);
+      assert.ok(result.thursdayShift > 0);
       assertNoPageErrors(page, assert);
     });
   });