Centre tab titles and close buttons in the tab bar

[dayglojesus]

Closes #37.

The bug

The tab title and close button were positioned by pinning their bottom edges 3pt and 4pt above the bottom of the tab:

[self addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:[close]-(4)-|" ...]];
[self addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:[title]-(3)-|" ...]];

Those insets centre a 16pt label and a 13pt button only when the tab is 22pt tall, which is what OakTabBarView.mm:720's intrinsicContentSize asks for.

It never gets 22pt. The tab bar is an NSTitlebarAccessoryViewController view (DocumentWindowController.mm:208-212), and AppKit fixes those at a height of its own choosing. Probing it on macOS 26.5 (25F71):

requested intrinsic height -> actual
    10 -> 36.0        36 -> 36.0
    23 -> 36.0        60 -> 36.0

The request is discarded entirely — it is neither a floor nor a ceiling. No window configuration moves it either: titlebarAppearsTransparent, titleVisibility, NSWindowStyleMaskFullSizeContentView, and both unified toolbar styles all give 36. Only layoutAttribute Right or Left differ (32pt), and those place the accessory inline beside the window title, which is no use for a tab bar.

So the accessory is 36pt, tabs fill it at 35pt (NSHeight(self.bounds)-1), and the two bottom-pinned subviews sat well below the middle of them.

Measurements

Clear space above and below the title ink, in device pixels, in a 70px tab, taken from window captures at 2x:

	above	below
before	38	7
after	26	25

The close button measures 28/28 after the change. Both figures land within a pixel of tagliala's build, which is the reference the issue points at.

The change

Both subviews are constrained to the tab's centre, which holds at whatever height AppKit hands us, with a 1pt constant so they sit fractionally low rather than dead centre — that reads better at this tab height.

Relationship to the upstream PRs

The issue links two PRs on tagliala's fork.

• tagliala#25 (merged there) is the same approach as this. Its diff does not apply here: it deletes V:[close]-(10)-| and V:[title]-(9)-|, values that fork introduced as a workaround with a TODO admitting as much. This tree still has the original 4 and 3.
• tagliala#16 (still open there) removes the ±1 in setCurrentLayout's frame arithmetic. It does not touch the pinning, so it does not address the cause; applied on its own here it moves the title 0.5pt further from centre and the close button 1pt. Not taken.

Worth noting separately

intrinsicContentSize returning 23 has no effect on the rendered height and will mislead the next person to read it. tagliala's issue #15 ("the tab navigation bar is too tall") is unfixable from that direction for the same reason — they observed that changing intrinsicContentSize did nothing, and this is why. Left alone here to keep the change to the reported bug.