Fix chart viewport hiding after timeframe switch.

Use pendingViewportEpoch so tail refresh applies default range on new timeframe instead of reusing the previous period's saved range.

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

manual_trading_hub/static/chart.js

@@ -178,6 +178,7 @@
   let chartRangeUserLocked = false;
   let chartRangeLockTimer = null;
   let suppressRangeUserLock = false;
+  let pendingViewportEpoch = -1;
   let priceTagTimer = null;
   let tfDigitBuf = "";
   let tfDigitTimer = null;
@@ -2032,6 +2033,7 @@
   }
 
   function markChartRangeUserAdjusted() {
+    pendingViewportEpoch = -1;
     chartRangeUserLocked = true;
     if (chartRangeLockTimer) clearTimeout(chartRangeLockTimer);
     chartRangeLockTimer = setTimeout(function () {
@@ -2248,16 +2250,27 @@
           applyChartPriceFormat();
         }
       }
+      const keepViewport =
+        pendingViewportEpoch !== chartViewEpoch &&
+        savedRange &&
+        isVisibleRangeValidForCandles(savedRange, candleCountBefore);
       applyCandlesToChart(
         mergeCandles(lastCandles, alignCandlesToTick(data.candles), { prepend: false }),
         0,
-        { preserveRange: true, skipAutoScale: chartRangeUserLocked }
+        {
+          preserveRange: !!keepViewport,
+          skipAutoScale: chartRangeUserLocked,
+        }
       );
       if (epochAtStart !== chartViewEpoch) return;
       const n = lastCandles.length;
-      if (!restoreVisibleLogicalRange(savedRange, n)) {
+      if (pendingViewportEpoch === chartViewEpoch) {
+        applyDefaultVisibleRange();
+      } else if (savedRange && isVisibleRangeValidForCandles(savedRange, n)) {
+        restoreVisibleLogicalRange(savedRange, n);
+      } else if (!chartRangeUserLocked) {
         const curRange = chart && chart.timeScale().getVisibleLogicalRange();
-        if (!chartRangeUserLocked && curRange && !isVisibleRangeValidForCandles(curRange, n)) {
+        if (!curRange || !isVisibleRangeValidForCandles(curRange, n)) {
           restoreVisibleLogicalRange(tailVisibleLogicalRange(n), n);
         }
       }
@@ -2512,6 +2525,7 @@
     chartDataLoading = true;
     if (resetView) {
       chartViewEpoch += 1;
+      pendingViewportEpoch = chartViewEpoch;
       chartRangeUserLocked = false;
       if (chartRangeLockTimer) {
         clearTimeout(chartRangeLockTimer);