What is expected behavior in this case:

configs = new ConfigArray([
	{
		files: ['x/*.js'],
		defs: {
			severity: 'error'
		}
	}
], { basePath: '', schema });

configs.normalizeSync();

const config = configs.getConfig('/x/foo.js'); // `undefined` or `{ defs: { severity: 'error' } }` ?

Interesting question. My initial reaction is that this should return undefined, because the match is done from the left of the string, and because the pattern doesn't start with /, that isn't a match. I've added some tests to this effect.

This would change how Linter works in browsers, where cwd is / (at least the way we bundle).

When I add the following to src/playground/index.js:

import { Linter } from "./node_modules/eslint/lib/linter/";

const linter = new Linter({ configType: "flat" });
const code = "foo();";
const config = [{
    files: ["bar/baz.js"],
    rules: {
        "no-undef": 2
    }
}];
const filename = "/bar/baz.js";
const result = linter.verify(code, config, filename);
console.log(JSON.stringify(result, null, 4));

currently, it logs:

[
    {
        "ruleId": "no-undef",
        "severity": 2,
        "message": "'foo' is not defined.",
        "line": 1,
        "column": 1,
        "nodeType": "Identifier",
        "messageId": "undef",
        "endLine": 1,
        "endColumn": 4
    }
]

because the config matches. However, after this change the config doesn't match and it logs:

[]

To me, files: ['**/*.js'] matching '/x/foo.js' is confusing, as file patterns are supposed to be relative to something. If we don't want basePath to default to an absolute path (currently, it basically defaults to cwd), then when basePath isn't passed I'd rather expect this to work in "relative-only" mode and don't match any absolute paths.

Also, note that after this change process.cwd() would still be implicitly used when basePath is a non-empty non-absolute path, which looks inconsistent with how empty basePath is treated.

const configs = new ConfigArray([
	{
		files: ['**/*.js'],
		defs: {
			severity: 'error'
		}
	}
], { basePath: 'foo', schema }); // works the same as if it were `path.resolve('foo')`

configs.normalizeSync();

console.log(configs.getConfig('/foo/bar.js')); // undefined
console.log(configs.getConfig(path.resolve('foo/bar.js'))); // { defs: { severity: 'error' } }

To me, files: ['**/*.js'] matching '/x/foo.js' is confusing, as file patterns are supposed to be relative to something. If we don't want basePath to default to an absolute path (currently, it basically defaults to cwd), then when basePath isn't passed I'd rather expect this to work in "relative-only" mode and don't match any absolute paths.

I need some context for this: are you saying this is the way things work in ESLint currently or after the change in this PR?