[Feature]: Configurable default grouping for the HTML report ("Merge files" as a default)

[jonrohan]

🚀 Feature Request

Let the HTML reporter's grouping default be set at report-generation time (via config, and/or a URL param), so a report can open already grouped by describe, i.e. the equivalent of the "Merge files" toggle being on, without each viewer flipping it by hand.

// playwright.config.ts
reporter: [['html', { groupBy: 'suite' }]]

Motivation

I have a monorepo where a lot of tests are generated inside a single shared spec file, and each unit is wrapped in test.describe('@scope/<package>'). In the merged HTML report every test collapses under that one file header, so I can't tell which package a failure belongs to at a glance.

The "Merge files" option (from #38870) already does exactly the grouping I want: it groups by describe, and the group header becomes my @scope/<package>. The catch is that it's a per-viewer setting stored in localStorage['mergeFiles'], so:

• CI-published reports always open ungrouped, and every person has to know about and click "Merge files".
• It can't be shared via a link (it's a stored setting, not a URL param).
• There's no way to bake the default into the report we publish as an artifact.

What I'm asking for

A supported way to set the default grouping when the report is generated, e.g. an HtmlReporterOptions field (groupBy: 'file' | 'suite', defaulting to 'file' for back-compat), threaded into the report so it opens grouped. A URL param would be a nice bonus for sharing. The per-viewer toggle would still override it.

The plumbing looks mostly there already: createMergedFilesModel in reportView.tsx groups by test.path[0], gated behind useSetting('mergeFiles', ...). This would let the generated report seed that default.

(Note: #38870 was closed into #21202, which is broader metadata filtering/grouping. This is narrower, just making the existing "Merge files" grouping configurable as a default, so I'm filing separately. Happy to have it folded in if you'd prefer.)