WebView state unexpectedly persists across sessions without a WebviewPanelSerializer #240207

In the docs at https://code.visualstudio.com/api/references/vscode-api#WebviewPanelSerializer<T>, it says:

There are two types of webview persistence:

• Persistence within a session.
• Persistence across sessions (across restarts of the editor).

A WebviewPanelSerializer is only required for the second case: persisting a webview across sessions.

The suggestion is that without a WebviewPanelSerializer, state is not persisted across sessions.

However, that's not what I'm seeing. I don't have a WebviewPanelSerializer but my state is persisting across restarts. This can be reproduced by opening the webview-view-sample project and making the following change:

// ORIGINAL CODE
				<script nonce="${nonce}" src="${scriptUri}"></script>
// NEW CODE
				<script nonce="${nonce}">
				const vscode = acquireVsCodeApi();
				const originalState = vscode.getState() ?? {};

				console.log(\`State log: \${originalState.log?.join("\\n")}\`);
				vscode.setState({
					...(originalState ?? {}),
					log: [...(originalState.log ?? []), \`Webview state added at ${new Date()}\`]
				});
				</script>

Loaded the webview, run "Reload Webviews" a few times, then shut down VS Code. Then re-opened VS Code and open the webview and you'll see the following:

I would've expected any state from previous launches of VS Code to be gone.

As an aside, things that aren't clear to me from the limited docs:

• is the intention that state is lost in VS Code restart, or extension host restart? What about "Reload Window"?
• is state shared across all webviews, or scoped by an id?
• does setState always replace the entire object, or is there a way to add just new fields (I ask because if the state is shared amongst all webviews, that could result in them overwriting each others state)

Does this issue occur when all extensions are disabled?: Yes

Version: 1.97.0 (user setup)
Commit: 33fc5a9
Date: 2025-02-04T22:41:26.688Z
Electron: 32.2.7
ElectronBuildId: 10660205
Chromium: 128.0.6613.186
Node.js: 20.18.1
V8: 12.8.374.38-electron.0
OS: Windows_NT x64 10.0.26100

I previously raised this at #236494 but it was closed asking for more info, however I added more info over a month ago but the issue remains closed and it's not clear how else this issue can be progressed. If it's better to reopen that issue and close this one, that's fine by me. Thanks!