The Day Vertical Split View Tabs Refused to Behave

This was one of those bugs that looks tiny in a ticket, but feels huge when you see it in the UI.

I was working on Bug 2029684 (reviewed in D292576) where vertical Split View tabs could look either too big or too small.

No crash.
No red error.
Just a UI that felt slightly wrong depending on the state.

And honestly, those are sometimes the hardest bugs to ignore.

What made it interesting

The issue showed up most in vertical/collapsed Split View scenarios.

In one state, sizing looked roomy.
In another, it looked compressed.

The tabs were technically there and working, but the visual rhythm was off and that affects the whole experience.

Digging into it

I traced the behavior in:

• browser/themes/shared/tabbrowser/tabs.css

At first I thought I was going to adjust spacing.
But the real question became:

Is the tab item allowed to shrink correctly inside this container?

That led to a very focused fix:

• set min-width: 0 on .tabbrowser-tab in the affected vertical Split View path

Why one line helped so much

Sometimes default sizing rules quietly fight your layout.

You tell the container to be constrained, but the child still holds onto width it does not need.
That is when UI starts to feel inconsistent across states.

With min-width: 0, the sizing behavior becomes explicit and cooperative.
The tab can actually respect the available space.

Review and landing

This one moved quickly:

• created in Phabricator
• reviewed by kcochrane
• accepted
• landed as rFIREFOXAUTOLAND3d5fe183491d

I like this kind of patch flow small scope, clear impact, clean review.

What I took from this

This bug reminded me that polish is part of engineering.

A tiny CSS line can remove a lot of visual friction for users when it is placed in the right context.

And more importantly:

“Too big” or “too small” is often not about taste. It is usually a layout rule that needs to be made explicit.

Closing

D292576 was small on paper, but meaningful in practice.

It made Firefox’s vertical Split View feel more stable, and it gave me another reminder that good frontend work is often about quiet precision.