This solution works well for horizontal tabs but it's odd that we don't have the drop indicator for vertical tabs; I think it was just overlooked during the initial work to support vertical tabs. I'm sure there's a bug on file for it but if its low effort to make this work for both orientations it would make sense to do so.

r+ for sidebar changes

Lgtm, thanks!

@emilio already submitted a patch for this D281094 and he is assigned to the bug so I'm confused why you submitted a patch also.

This looks great! One thing that doesn't look quite right is when dragging two splitview wrappers to a new window. It adds all 4 tabs but doesn't preserve the splitview (whereas dragging 1 splitview wrapper does preserve it in the new window). I think you need to make sure adoptSplitview is being called here: https://searchfox.org/firefox-main/rev/8d8cb89b010e9214fdb87a915511f265eeb06530/browser/components/tabbrowser/content/tabbrowser.js#3811

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I had a few fly-by questions and thoughts. I'm not fully dialed into the why's behind your archictural changes and if you have a doc about the implemenation I'd love to take a look. :)

Apologies for the delay, this looks okay to me although I couldn't apply the patch so you likely need to rebase.

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Thanks for reworking some of this. Looks good to me.

Looks good from my perspective - what we discussed with @daleharvey

Lgtm, thanks!

I have a few more questions :) Happy to jump on a call so we can talk through this if that's easier.

Lgtm, thanks!

This is working great! I have a few questions and requests for changes but I think this is nearly there.

This patch handles the edge cases in the bug regarding extensions but also keyboard navigation which I discovered while working on it. I know this is a lot of changes and I'm happy to do a demo/walk through of it for anyone who picks up review of this. If you pull down the code then you see the behavior changes just calling gBrowser.moveTabForward() or gBrowser.moveTabBackward() in the browser toolbox.

In D278107#9632137, @jsudiaman wrote:

Now that I've created a new folder for splitview, maybe it'd make sense to some of the splitview specific modules in tabbrowser/content into this folder?

That makes sense, yeah. Should we do this in a follow-up?

Now that I've created a new folder for splitview, maybe it'd make sense to some of the splitview specific modules in tabbrowser/content into this folder? Could follow the same pattern with tab groups.

Sidebar changes lgtm, thanks!

I'll be on PTO for the next two weeks so @jsudiaman can take over review.

In D276409#9578232, @sfoster wrote:

In D276409#9577666, @ayeddi wrote:

@sfoster thank you for the review request? Could you please clarify what are your expectations of the a11y review?

Oh sorry I should have been clearer. The review request was just for the changes I needed to make to the test_tabbrowser.xhtml test.

TBH we have never had a request to fully reviewed the a11y of the sidebar, only the GenAI panel was tested a few months ago.

We should follow up on this. The plan - and this patch - is to enable the new sidebar by default for nightly only.

In D270178#9577192, @nsharpley wrote:

There is one small bug that I'm happy to leave as a follow up. When dragging the split view wrapper out of/within a tab group, only one of the split view tabs has the tab group underline. See video. I think both should?

Screen Recording 2025-12-16 at 10.18.32 AM.mov18 MBDownload

testing-exception-elsewhere: Tests already existing for this feature in the tabbrowser directory.

Lgtm, thanks!

Lgtm, thanks!

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