fix(positioning): prevent canvas collapse on type/range toggle

- Lock #positioning-canvas min-height to current offsetHeight before each
  HTMX swap, release it in htmx:afterSwap — prevents flash-to-zero during
  Chart.js initialization in the new content
- Add CSS min-height:200px fallback on all canvas containers so they never
  fully collapse even before JS runs
- Extract _swapCanvas() helper to deduplicate setRange/setType logic

Root cause of visual collapse: cot_positioning_combined table missing
(needs sqlmesh plan prod + export_serving to materialize).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/src/beanflows/dashboard/templates/positioning.html

@@ -72,24 +72,29 @@ function _positioningUrl() {
   return POSITIONING_URL + '?range=' + currentRange + '&type=' + currentType;
 }
 
+function _swapCanvas(url) {
+  var el = document.getElementById('positioning-canvas');
+  el.style.minHeight = el.offsetHeight + 'px';
+  el.classList.add('canvas-loading');
+  htmx.ajax('GET', url, { target: '#positioning-canvas', swap: 'innerHTML' });
+}
+
 function setRange(val) {
   currentRange = val;
   var url = _positioningUrl();
   window.history.pushState({}, '', url);
-  document.getElementById('positioning-canvas').classList.add('canvas-loading');
-  htmx.ajax('GET', url, { target: '#positioning-canvas', swap: 'innerHTML' });
+  _swapCanvas(url);
 }
 
 function setType(val) {
   currentType = val;
   var url = _positioningUrl();
   window.history.pushState({}, '', url);
-  document.getElementById('positioning-canvas').classList.add('canvas-loading');
-  htmx.ajax('GET', url, { target: '#positioning-canvas', swap: 'innerHTML' });
   // Sync type pill active state immediately (canvas swap will also re-sync)
   document.querySelectorAll('#type-pills .filter-pill').forEach(function (btn) {
     btn.classList.toggle('active', btn.textContent.trim() === (val === 'combined' ? 'F+O Combined' : 'Futures'));
   });
+  _swapCanvas(url);
 }
 
 // MA toggles: client-side only — update Chart.js dataset visibility
@@ -103,7 +108,9 @@ function toggleMA(key) {
 
 document.addEventListener('htmx:afterSwap', function (e) {
   if (e.detail.target.id === 'positioning-canvas') {
-    document.getElementById('positioning-canvas').classList.remove('canvas-loading');
+    var el = document.getElementById('positioning-canvas');
+    el.classList.remove('canvas-loading');
+    el.style.minHeight = '';   // release height lock
     requestAnimationFrame(function() {
       ['priceChart', 'cotChart'].forEach(function(id) {
         var c = Chart.getChart(id);

web/src/beanflows/static/css/input.css

@@ -736,6 +736,13 @@
     transition: opacity 0.15s;
   }
 
+  /* Prevent canvas from collapsing to 0 height during HTMX swaps.
+     Chart.js needs the container to have non-zero height before it can
+     measure the aspect ratio. min-height is locked in JS before each swap. */
+  #positioning-canvas, #supply-canvas, #warehouse-canvas, #weather-canvas {
+    min-height: 200px;
+  }
+
   /* Freshness badges */
   .freshness-bar {
     display: flex;