src/vs/workbench/contrib/search/browser/symbolsQuickAccess.ts/SymbolsQuickAccessProvider/defaultFilterValue
class SymbolsQuickAccessProvider:	get defaultFilterValue(): string | undefined {

		// Prefer the word under the cursor in the active editor as default filter
		const editor = this.codeEditorService.getFocusedCodeEditor();
		if (editor) {
			return withNullAsUndefined(getSelectionSearchString(editor));
		}

		return undefined;
	}