fix: Fix bug that caused blocks to be inserted into an immovable stack.

src/navigation.ts

@@ -429,9 +429,28 @@ export class Navigation {
         return connection as Blockly.RenderedConnection;
       }
 
-      // 2. Connect statement blocks to next connection.
+      // 2. Connect statement blocks to next connection. Only return a next
+      // connection to which the statement block can actually connect; some
+      // may be ineligible because they are e.g. in the middle of an immovable
+      // stack.
       if (stationaryNode.nextConnection && !movingHasOutput) {
-        return stationaryNode.nextConnection;
+        let nextConnection: Blockly.RenderedConnection | null =
+          stationaryNode.nextConnection;
+        while (nextConnection) {
+          if (
+            movingBlock.workspace.connectionChecker.canConnect(
+              movingBlock.previousConnection,
+              nextConnection,
+              true,
+              Infinity,
+            )
+          ) {
+            return nextConnection;
+          }
+          nextConnection =
+            nextConnection.getSourceBlock().getNextBlock()?.nextConnection ??
+            null;
+        }
       }
 
       // 3. Output connection. This will wrap around or displace.

test/webdriverio/test/insert_test.ts

@@ -5,6 +5,7 @@
  */
 
 import * as chai from 'chai';
+import * as Blockly from 'blockly';
 import {Key} from 'webdriverio';
 import {
   getFocusedBlockType,
@@ -103,4 +104,32 @@ suite('Insert test', function () {
       await getFocusedBlockType(this.browser),
     );
   });
+
+  test('Does not insert between immovable blocks', async function () {
+    // Focus the create canvas block; we want to ensure that the newly
+    // inserted block is not attached to its next connection, because doing
+    // so would splice it into an immovable stack.
+    await focusOnBlock(this.browser, 'create_canvas_1');
+    await this.browser.execute(() => {
+      Blockly.getMainWorkspace()
+        .getAllBlocks()
+        .forEach((b) => b.setMovable(false));
+    });
+    await tabNavigateToToolbox(this.browser);
+
+    // Insert 'if' block
+    await keyRight(this.browser);
+    // Choose.
+    await sendKeyAndWait(this.browser, Key.Enter);
+    // Confirm position.
+    await sendKeyAndWait(this.browser, Key.Enter);
+
+    // Assert inserted inside first block p5_setup not at top-level.
+    chai.assert.equal('controls_if', await getFocusedBlockType(this.browser));
+    await keyUp(this.browser);
+    chai.assert.equal(
+      'p5_background_color',
+      await getFocusedBlockType(this.browser),
+    );
+  });
 });