extensions/markdown-language-features/src/preview/previewManager.ts/MarkdownPreviewManager/trackActive
class MarkdownPreviewManager:	private trackActive(preview: ManagedMarkdownPreview): void {
		preview.onDidChangeViewState(({ webviewPanel }) => {
			this.setPreviewActiveContext(webviewPanel.active);
			this._activePreview = webviewPanel.active ? preview : undefined;
		});

		preview.onDispose(() => {
			if (this._activePreview === preview) {
				this.setPreviewActiveContext(false);
				this._activePreview = undefined;
			}
		});
	}