trendline extension slope fix

docs/implementation.md

@@ -925,6 +925,12 @@ Worth generalising: a measurement that looks absurd is worth chasing rather than
 clamping. The absurd number here was "67 bars of padding", and the bug behind it
 had nothing to do with padding.
 
+The same median must govern every conversion past the final bar, not only the
+future slots. Leaving `timeAtIndex()` and `indexAt()` on their final raw gap
+made the first future 5m slot after the 17:00-18:00 settlement break advance a
+trendline by 65 minutes. Source-timeframe lines therefore became thirteen times
+steeper immediately beyond the live edge. All three paths now use `barInterval`.
+
 ### 2026-08-14 — trendlineproblem: high-TF lines used the wrong bar grid
 
 **Confirmed and fixed behind `TRENDLINE_SOURCE_GEOMETRY`.** No drawing records

static/chart.js

@@ -1661,7 +1661,10 @@ class ConfluenceChart {
     }
     const last = bars.length - 1;
     if (time >= bars[last].t) {
-      const step = bars[last].t - bars[last - 1].t;
+      // Future slots use the median interval, not the tail gap. After the CME
+      // settlement break the final gap is 65 minutes, and using it here makes a
+      // 5m future projection advance thirteen bars per visible slot.
+      const step = ConfluenceChart.barInterval(bars);
       return last + (step ? (time - bars[last].t) / step : 0);
     }
     let lo = 0;
@@ -1759,7 +1762,8 @@ class ConfluenceChart {
     if (index <= 0) return Math.round(bars[0].t + index * (bars[1].t - bars[0].t));
     const last = bars.length - 1;
     if (index >= last) {
-      return Math.round(bars[last].t + (index - last) * (bars[last].t - bars[last - 1].t));
+      const step = ConfluenceChart.barInterval(bars);
+      return Math.round(bars[last].t + (index - last) * step);
     }
     const lower = Math.floor(index);
     const fraction = index - lower;

tests/e2e/trendline.test.mjs

@@ -106,6 +106,58 @@ test('a 30m line keeps source geometry when the 1m window starts after its ancho
       assert.equal(await page.locator('.chart-line-hit').evaluate(node => getComputedStyle(node).fill),
         'none', 'the invisible polyline hit region filled the chart as a black polygon');
       assertNoPageErrors(page, assert);
+  });
+});
+
+test('future 5m projections keep their slope after the settlement break',
+  { timeout: 180000 }, async () => {
+    await withChart(async page => {
+      const result = await page.evaluate(() => {
+        const c = window.__chart;
+        const fiveMinutes = 5 * 60;
+        const thirtyMinutes = 30 * 60;
+        // The final 65-minute gap is the 17:00-18:00 CME settlement break.
+        const displayed = Array.from({ length: 30 }, (_, index) => ({
+          tf: '5m', t: index * fiveMinutes, o: 100, h: 101, l: 99, c: 100, v: 1, closed: true,
+        }));
+        displayed.push({
+          tf: '5m', t: displayed.at(-1).t + 13 * fiveMinutes,
+          o: 100, h: 101, l: 99, c: 100, v: 1, closed: true,
+        });
+        const line = {
+          id: 'settlement-gap', kind: 'manual', tf: '30m',
+          anchor_t: 0, anchor_p: 100, last_t: thirtyMinutes,
+          slope: 6 / thirtyMinutes, cutoff_t: null,
+        };
+        c.bars = displayed;
+        c.setTrendlineGeometry({
+          mode: 'source_tf',
+          series: {
+            '30m': {
+              times: [0, thirtyMinutes, 2 * thirtyMinutes, 3 * thirtyMinutes,
+                4 * thirtyMinutes, 7 * thirtyMinutes],
+              duration: thirtyMinutes,
+              future_times: [8 * thirtyMinutes, 9 * thirtyMinutes],
+            },
+          },
+        });
+        const last = displayed.length - 1;
+        const before = c.linePriceAtIndex(line, last - 1);
+        const current = c.linePriceAtIndex(line, last);
+        const future = c.linePriceAtIndex(line, last + 1);
+        return {
+          historicalChange: current - before,
+          futureChange: future - current,
+          futureTime: c.timeAtIndex(last + 1),
+          expectedFutureTime: displayed[last].t + fiveMinutes,
+        };
+      });
+
+      assert.equal(result.futureTime, result.expectedFutureTime,
+        'one future 5m slot inherited the 65-minute settlement gap');
+      assert.ok(Math.abs(result.futureChange - result.historicalChange) < 1e-9,
+        `future slope changed from ${result.historicalChange} to ${result.futureChange}`);
+      assertNoPageErrors(page, assert);
     });
   });