fix: preserve z-indexed subplots during relayout for correct pan/zoom

When traces have different `zorder` values, plotly.js creates z-indexed
subplots (e.g., `xyz2`, `xyz3`) in `drawFramework`. However, when
`relayout` is called (e.g., during resize), `supplyDefaults` resets
`_plots` via `linkSubplots`, losing these z-indexed subplots. Since
`relayout` doesn't trigger `drawFramework`, they aren't recreated.

This caused pan/zoom to fail on the first attempt in react-plotly.js,
because `updateSubplots` in `dragbox.js` couldn't find the z-indexed
subplots to transform.

Fix:
1. In `linkSubplots`, preserve z-indexed subplots from `oldSubplots`
2. In `updateSubplots`, include z-indexed subplots from `_plots`

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: ryan-williams

src/plots/cartesian/dragbox.js

@@ -872,9 +872,18 @@ function makeDragBox(gd, plotinfo, x, y, w, h, ns, ew) {
     function updateSubplots(viewBox) {
         var fullLayout = gd._fullLayout;
         var plotinfos = fullLayout._plots;
-        var subplots = fullLayout._subplots.cartesian;
+        var subplots = fullLayout._subplots.cartesian.slice();
         var i, sp, xa, ya;
 
+        // Include z-indexed subplots from _plots that may not be in _subplots.cartesian
+        // (e.g., after a relayout that resets _subplots.cartesian but preserves _plots)
+        var zindexSeparator = constants.zindexSeparator;
+        for(var plotId in plotinfos) {
+            if(plotId.indexOf(zindexSeparator) !== -1 && subplots.indexOf(plotId) === -1) {
+                subplots.push(plotId);
+            }
+        }
+
         if(hasSplom) {
             Registry.subplotsRegistry.splom.drag(gd);
         }

src/plots/plots.js

@@ -14,6 +14,7 @@ var Color = require('../components/color');
 var BADNUM = require('../constants/numerical').BADNUM;
 
 var axisIDs = require('./cartesian/axis_ids');
+var cartesianConstants = require('./cartesian/constants');
 var clearOutline = require('../components/shapes/handle_outline').clearOutline;
 var scatterAttrs = require('../traces/scatter/layout_attributes');
 
@@ -895,6 +896,19 @@ plots.linkSubplots = function(newFullData, newFullLayout, oldFullData, oldFullLa
         }
     }
 
+    // Preserve z-indexed subplots (e.g. 'xyz2', 'xyz3') from oldSubplots to _plots only.
+    // These are created by drawFramework when traces have different zorder values.
+    // We preserve them to _plots (not _subplots.cartesian) so that drag/pan operations
+    // in dragbox.js can still transform them during relayout operations that don't
+    // trigger a full redraw (e.g., resize). If drawFramework runs later, it will
+    // properly update/remove these subplots as needed.
+    var zindexSeparator = cartesianConstants.zindexSeparator;
+    for(var oldId in oldSubplots) {
+        if(oldId.indexOf(zindexSeparator) !== -1 && !newSubplots[oldId]) {
+            newSubplots[oldId] = oldSubplots[oldId];
+        }
+    }
+
     // while we're at it, link overlaying axes to their main axes and
     // anchored axes to the axes they're anchored to
     var axList = axisIDs.list(mockGd, null, true);