Fix the file browser default column width

[jtpio]

In #7423 we tried to make the file browser default column width stable.

But it looks like they are not by default, as reloading the page several times in a row would yield different widths for the columns.

The UI tests have added a maxDiffPixels to get around that for now just to be able to get a release out, here:

notebook/ui-tests/test/mobile.spec.ts

Lines 38 to 40 in 324de44

	expect(await page.screenshot()).toMatchSnapshot('tree.png', {
	maxDiffPixels: 500,
	});

and here:

notebook/ui-tests/test/settings.spec.ts

Lines 36 to 38 in 324de44

	expect(await page.screenshot()).toMatchSnapshot('top-hidden.png', {
	maxDiffPixels: 500,
	});

But we should clearly undo that change because the file browser should always open the same way.

cc @krassowski @JasonWeill I would appreciate if you could help have a look at this since you both worked on the upstream feature in JupyterLab. Thanks!