Sample trendlines on their own timeframe and hit-test the painted line.

1m was putting a point on every bar for 30m/1d lines, which kinked at
tape holes and made hover/select miss. Clicks that were eaten by the
vertical pan gesture now select on pointer-up.

AGENTS.md

@@ -250,8 +250,14 @@ cost on the hot path unless the screen or an alert has to change.
   client needs. Caps are named settings, not leftover literals.
 - Crosshair move is for the cursor (OHLC, drawing tooltip). Do not
   rebuild overlays that only depend on the viewport.
-- Overlay redraws go through `scheduleOverlays`. Force when data or
-  size changed; let the range-key skip no-ops.
+- Overlay redraws go through `scheduleOverlays`. Force when bars are
+  replaced, a new bar opens, or size changed. **A forming-bar tick is
+  not that.** `updateBar` must not `scheduleOverlays(true)` on every
+  tick — that walked every trendline across 5k bars at ~4 Hz and
+  melted the tab. We wrote this section and then shipped that anyway.
+- Diagnostic `getImageData` / painted-pixel sampling is never on the
+  live overlay path. `?diag=1` is not a license to sync the GPU every
+  frame.
 - `window` pointermove/up attach only while a tool is armed or a drag
   is live. Do not leave them on for the life of the page.
 - Prefer one rAF over N DOM rebuilds. Read `offsetWidth` only if the

docs/implementation.md

@@ -1182,6 +1182,23 @@ Context labels now update only with the view. All overlay redraws share
 one rAF and skip if the logical/price range did not change. Pointermoves
 attach only for an armed tool or an in-progress drag.
 
+### 2026-08-26 — we wrote Stay cheap, then melted the tab anyway
+
+`AGENTS.md` already said: nothing extra on the hot path unless the
+screen has to change. The next change put `scheduleOverlays(true)` in
+`updateBar`, so every Schwab tick (~4 Hz) rebuilt handles, bridges,
+comments and labels — walking every trendline across 5k bars. With
+`?diag=1` that same path called `getImageData` on the chart canvases
+and synced the GPU every time.
+
+The tab pegged a core. It looked like “the frontend is just heavy.”
+It was a forming-bar tick treated as an overlay rebuild.
+
+A forming tick updates the last candle only. Force overlays when bars
+are replaced, a new bar opens, or size changed. Painted-pixel sampling
+is a capture-time measurement, not a live diagnostic. The rule is now
+named in `AGENTS.md` with this failure, not a vague “when data changed.”
+
 ### 2026-08-16 — the rest of the same session, none of it subtle
 
 Written because they were skipped the first time and then asked for.

static/app.js

@@ -695,7 +695,7 @@ createApp({
     }
 
     function handleChartClick(param) {
-      const id = chartApi.hitTest(param);
+      const id = chartApi.hitTest(param, { painted: param.painted });
       selectedDrawing.value = id;
       selectedDrawings.value = id ? [id] : [];
     }

static/chart.js

@@ -1866,8 +1866,16 @@ class ConfluenceChart {
   }
 
   finishToolGesture(event) {
+    const tap = this.mousePan && this.mousePan.minValue == null;
     this.mousePan = null;
     if (!this.armedTool) this.detachPointers('tool');
+    if (tap && this.onChartClick) {
+      const point = this.eventPoint(event);
+      if (point && this.withinPlot(point) && point.t != null) {
+        this.suppressClickUntil = Date.now() + 400;
+        this.onChartClick({ point, time: point.t, painted: true });
+      }
+    }
     if (!this.armedTool || !this.gesture) return;
     const tool = this.armedTool;
     const { start, end } = this.gesture;
@@ -2266,7 +2274,31 @@ class ConfluenceChart {
     this.tooltip.style.top = `${Math.max(8, param.point.y - 30)}px`;
   }
 
-  hitTest(param) {
+  paintedLineY(level, time, x) {
+    const data = this.levelSeries.get(level.id)?.series.data();
+    if (!data?.length) return null;
+    const stamp = time == null ? this.chart.timeScale().coordinateToTime(x) : time;
+    if (stamp == null) return null;
+    const t = Number(stamp);
+    let lo = 0;
+    let hi = data.length - 1;
+    if (t <= Number(data[0].time)) return this.candles.priceToCoordinate(data[0].value);
+    if (t >= Number(data[hi].time)) return this.candles.priceToCoordinate(data[hi].value);
+    while (hi - lo > 1) {
+      const mid = (lo + hi) >> 1;
+      if (Number(data[mid].time) <= t) lo = mid;
+      else hi = mid;
+    }
+    const left = data[lo];
+    const right = data[hi];
+    const span = Number(right.time) - Number(left.time);
+    const price = span
+      ? left.value + (right.value - left.value) * (t - Number(left.time)) / span
+      : left.value;
+    return this.candles.priceToCoordinate(price);
+  }
+
+  hitTest(param, options = {}) {
     let best = null;
     const visible = this.visibleBarIndexes();
     if (!visible) return null;
@@ -2278,16 +2310,9 @@ class ConfluenceChart {
     }
     for (const level of this.levels.filter(value =>
       value.kind === 'manual' && !ConfluenceChart.isFlat(value) && !value.hidden)) {
-      const startIndex = Math.max(this.indexAt(level.anchor_t), visible.from);
-      const naturalEnd = level.cutoff_t == null ? visible.to : this.indexAt(level.cutoff_t);
-      const endIndex = Math.min(naturalEnd, visible.to);
-      const pointIndex = this.indexAtCoordinate(param.point.x);
-      if (pointIndex == null || pointIndex < startIndex || pointIndex > endIndex) continue;
-      const linePrice = this.linePriceAtIndex(level, pointIndex);
-      const lineY = linePrice == null ? null : this.candles.priceToCoordinate(linePrice);
-      if (lineY == null) continue;
-      const distance = Math.abs(lineY - param.point.y);
-      if (distance <= 6 && (!best || distance < best.distance)) best = { id: level.id, distance };
+      const lineY = this.paintedLineY(level, param.time, param.point.x);
+      const distance = lineY == null ? Infinity : Math.abs(lineY - param.point.y);
+      if (distance <= 8 && (!best || distance < best.distance)) best = { id: level.id, distance };
     }
     for (const fib of this.fibs.filter(item => !item.hidden)) {
       const originX = this.coordinateAtTime(fib.anchor_t);
@@ -2469,9 +2494,18 @@ class ConfluenceChart {
     let points;
     if (this.trendlineGeometryMode === 'source_tf') {
       const cutoff = level.cutoff_t ?? Infinity;
-      points = this.bars
-        .filter(bar => bar.t >= level.anchor_t && bar.t <= cutoff)
-        .map(bar => ({ time: bar.t, value: this.sourceLinePriceAt(level, bar.t) }))
+      const stamps = new Set();
+      const source = this.sourceSeries(level);
+      for (const time of ConfluenceChart.sourceTimes(source) || []) {
+        if (time >= level.anchor_t && time <= cutoff) stamps.add(time);
+      }
+      stamps.add(level.anchor_t);
+      stamps.add(level.last_t);
+      const lastBar = this.bars[this.bars.length - 1]?.t;
+      if (lastBar != null && lastBar >= level.anchor_t && lastBar <= cutoff) stamps.add(lastBar);
+      points = [...stamps]
+        .sort((a, b) => a - b)
+        .map(time => ({ time, value: this.sourceLinePriceAt(level, time) }))
         .filter(point => point.value != null);
     } else {
       const start = this.indexAt(level.anchor_t);