Fix chart viewport regressions from tail refresh and period switch.

Remove pendingViewportEpoch, fetch only 30 tail bars on poll, restore wasViewingTail logic, and fix left-scroll range shift from actual merge delta.

Co-authored-by: Cursor <cursoragent@cursor.com>

manual_trading_hub/static/chart.js

@@ -178,7 +178,7 @@
   let chartRangeUserLocked = false;
   let chartRangeLockTimer = null;
   let suppressRangeUserLock = false;
-  let pendingViewportEpoch = -1;
+  const CHART_TAIL_REFRESH_LIMIT = 30;
   let priceTagTimer = null;
   let tfDigitBuf = "";
   let tfDigitTimer = null;
@@ -2033,7 +2033,6 @@
   }
 
   function markChartRangeUserAdjusted() {
-    pendingViewportEpoch = -1;
     chartRangeUserLocked = true;
     if (chartRangeLockTimer) clearTimeout(chartRangeLockTimer);
     chartRangeLockTimer = setTimeout(function () {
@@ -2168,6 +2167,7 @@
     });
     if (params.before_ms) qs.set("before_ms", String(params.before_ms));
     if (params.refresh) qs.set("refresh", "1");
+    if (params.tail) qs.set("tail", "1");
     const r = await fetch("/api/chart/ohlcv?" + qs.toString(), { credentials: "same-origin" });
     const data = await r.json();
     if (!r.ok) {
@@ -2197,8 +2197,10 @@
       if (data.exhausted) exhaustedLeft = true;
       const incoming = alignCandlesToTick(data.candles || []);
       if (!incoming.length) return;
-      const shift = incoming.length;
-      applyCandlesToChart(mergeCandles(lastCandles, incoming, { prepend: true }), shift);
+      const prevLen = lastCandles.length;
+      const merged = mergeCandles(lastCandles, incoming, { prepend: true });
+      const shift = merged.length - prevLen;
+      applyCandlesToChart(merged, shift);
       if (elStatus && !elStatus.classList.contains("err")) {
         elStatus.textContent =
           "已加载 " +
@@ -2230,12 +2232,15 @@
     const candleCountBefore = lastCandles.length;
     let savedRange = null;
     if (chart) savedRange = chart.timeScale().getVisibleLogicalRange();
+    const wasViewingTail =
+      !savedRange || isViewingChartTail(savedRange, candleCountBefore);
     try {
       const data = await fetchChartChunk({
         exchange_key: exKey,
         symbol: sym,
         timeframe: tf,
-        limit: chartChunkLimit(tf),
+        limit: CHART_TAIL_REFRESH_LIMIT,
+        tail: true,
       });
       if (myToken !== loadToken) return;
       if (vKey !== lastViewKey) return;
@@ -2250,25 +2255,28 @@
           applyChartPriceFormat();
         }
       }
-      const keepViewport =
-        pendingViewportEpoch !== chartViewEpoch &&
+      const shouldPreserve =
         savedRange &&
-        isVisibleRangeValidForCandles(savedRange, candleCountBefore);
+        isVisibleRangeValidForCandles(savedRange, candleCountBefore) &&
+        (chartRangeUserLocked || !wasViewingTail);
       applyCandlesToChart(
         mergeCandles(lastCandles, alignCandlesToTick(data.candles), { prepend: false }),
         0,
         {
-          preserveRange: !!keepViewport,
+          preserveRange: !!shouldPreserve,
           skipAutoScale: chartRangeUserLocked,
         }
       );
       if (epochAtStart !== chartViewEpoch) return;
       const n = lastCandles.length;
-      if (pendingViewportEpoch === chartViewEpoch) {
-        applyDefaultVisibleRange();
-      } else if (savedRange && isVisibleRangeValidForCandles(savedRange, n)) {
-        restoreVisibleLogicalRange(savedRange, n);
-      } else if (!chartRangeUserLocked) {
+      const minorTailUpdate = Math.abs(n - candleCountBefore) <= CHART_TAIL_REFRESH_LIMIT + 5;
+      if (wasViewingTail && !chartRangeUserLocked) {
+        restoreVisibleLogicalRange(tailVisibleLogicalRange(n), n);
+      } else if (shouldPreserve && minorTailUpdate) {
+        if (!restoreVisibleLogicalRange(savedRange, n) && !chartRangeUserLocked) {
+          restoreVisibleLogicalRange(tailVisibleLogicalRange(n), n);
+        }
+      } else if (!restoreVisibleLogicalRange(savedRange, n) && !chartRangeUserLocked) {
         const curRange = chart && chart.timeScale().getVisibleLogicalRange();
         if (!curRange || !isVisibleRangeValidForCandles(curRange, n)) {
           restoreVisibleLogicalRange(tailVisibleLogicalRange(n), n);
@@ -2525,7 +2533,6 @@
     chartDataLoading = true;
     if (resetView) {
       chartViewEpoch += 1;
-      pendingViewportEpoch = chartViewEpoch;
       chartRangeUserLocked = false;
       if (chartRangeLockTimer) {
         clearTimeout(chartRangeLockTimer);