Merge pull request matplotlib#27251 from QuLogic/webagg-resize

webagg: Don't resize canvas if WebSocket isn't connected

Author: ksunden

lib/matplotlib/backends/web_backend/js/mpl.js

@@ -192,6 +192,15 @@ mpl.figure.prototype._init_canvas = function () {
     }
 
     this.resizeObserverInstance = new this.ResizeObserver(function (entries) {
+        // There's no need to resize if the WebSocket is not connected:
+        // - If it is still connecting, then we will get an initial resize from
+        //   Python once it connects.
+        // - If it has disconnected, then resizing will clear the canvas and
+        //   never get anything back to refill it, so better to not resize and
+        //   keep something visible.
+        if (fig.ws.readyState != 1) {
+            return;
+        }
         var nentries = entries.length;
         for (var i = 0; i < nentries; i++) {
             var entry = entries[i];
@@ -239,7 +248,7 @@ mpl.figure.prototype._init_canvas = function () {
             // And update the size in Python. We ignore the initial 0/0 size
             // that occurs as the element is placed into the DOM, which should
             // otherwise not happen due to the minimum size styling.
-            if (fig.ws.readyState == 1 && width != 0 && height != 0) {
+            if (width != 0 && height != 0) {
                 fig.request_resize(width, height);
             }
         }