Change:
import { IIdentityProvider } from 'vs/base/browser/ui/list/list'; import { ObjectTree } from 'vs/base/browser/ui/tree/objectTree'; import { ITreeElement } from 'vs/base/browser/ui/tree/tree'; <del> import { IActionableTestTreeElement, TestExplorerTreeElement, TestItemTreeElement } from 'vs/workbench/contrib/testing/browser/explorerProjections/index'; </del> <ins> import { IActionableTestTreeElement, TestExplorerTreeElement, TestItemTreeElement, TestTreeErrorMessage } from 'vs/workbench/contrib/testing/browser/explorerProjections/index'; </ins> <del> export const testIdentityProvider: IIdentityProvider<TestItemTreeElement> = { </del> <ins> export const testIdentityProvider: IIdentityProvider<TestExplorerTreeElement> = { </ins> getId(element) { <del> return element.treeId + '0' + element.test.expand; </del> <ins> return element.treeId + '0' + (element instanceof TestTreeErrorMessage ? 'error' : element.test.expand); </ins> } };

Discussion:
<!-- Do Not Delete This! bugreporttemplate --<!-- Please read our Rules of Conduct: --<!-- Read our guide about submitting issues: --<!-- Search existing issues to avoid creating duplicates. --<!-- Test using the latest Insiders build to see if your issue has already been fixed: --<!-- Instead of creating your report here, use 'Report Issue' from the 'Help' menu in VS Code to pre-fill useful information. --<!-- Launch with to check. --Does this issue occur when all extensions are disabled?: N/A <!-- If you answered No above, use 'Help: Start Extension Bisect' from Command Palette to try to identify the cause. --<!-- Issues caused by an extension need to be reported directly to the extension publisher. The 'Help Report Issue' dialog can assist with this. --- VS Code Version: 1.69.0-insider OS Version: Windows 10 Steps to Reproduce: and install from VSIX the test-provider-sample from - this just sets instead of adding test cases. Testing panel and open outcome: shows "expanded arrow" but error message doesn't appear. Error appears in Developer Tools ! !

bootstrap- [uncaught exception]: TypeError: Cannot read properties of undefined (reading 'expand') onUnexpectedError @ bootstrap- (anonymous) @ bootstrap- emit @ node:events:390 window.onerror @ node:electron/js2c/rendererinit:73 platform1.globals.onerror @ bootstrap- TypeError: Cannot read properties of undefined (reading 'expand') at Object.getId (vscode-file://vscode-) at vscode-file://vscode- at (<anonymous) at IndexTreeModel.spliceSimple (vscode-file://vscode-) at IndexTreeModel.spliceSmart (vscode-file://vscode-) at (vscode-file://vscode-) at ObjectTreeModel.setChildren (vscode-file://vscode-) at ObjectTreeModel.setChildren (vscode-file://vscode-) at WorkbenchObjectTree.setChildren (vscode-file://vscode-) at NodeChangeList.applyTo (vscode-file://vscode-) at HierarchicalByLocationProjection.applyTo (vscode-file://vscode-) at TestingExplorerViewModel.applyProjectionChanges (vscode-file://vscode-) at (vscode-file://vscode-) at RunOnceScheduler.doRun (vscode-file://vscode-) at RunOnceScheduler.onTimeout (vscode-file://vscode-) onUnexpectedError @ bootstrap- (anonymous) @ bootstrap- emit @ node:events:390 window.onerror @ node:electron/js2c/rendererinit:73 platform1.globals.onerror @ Uncaught TypeError: Cannot read properties of undefined (reading 'expand') at Object.getId () at at (<anonymous) at IndexTreeModel.spliceSimple () at IndexTreeModel.spliceSmart () at () at ObjectTreeModel.setChildren () at ObjectTreeModel.setChildren () at WorkbenchObjectTree.setChildren () at NodeChangeList.applyTo () at HierarchicalByLocationProjection.applyTo () at TestingExplorerViewModel.applyProjectionChanges () at () at RunOnceScheduler.doRun () at RunOnceScheduler.onTimeout ()
I submitted a PR
/assign