feat(dock): make tab bar size configurable

[hipstersmoothie]

NOTE: This was completely written by AI but the issue is real. I was not able to find a way to set the height of the tab bar in any way.

Summary

• add DockSkin::tab_size and DockSkin::set_tab_size so applications can size dock-owned tab bars
• honor custom pixel sizes in Tab, including the outer height, inner surface, and proportional horizontal spacing
• add a rendered dock test proving a taller tab bar leaves correspondingly less panel content

Motivation

TabBar implements Sizable, but Size::Size(Pixels) currently falls through to the medium tab metrics. In addition, applications cannot pass a size to the TabBar constructed internally by DockSkin. This makes dock tabs clip when an application increases its global text scale.

The default remains Size::Medium, so existing docks are unchanged. Applications that need larger UI chrome can call:

skin.set_tab_size(px(40.), cx);

Testing

• cargo test -p gpui-component — 421 unit tests and all compatibility suites pass
• cargo fmt --all -- --check — passes
• cargo clippy -p gpui-component --all-targets — no diagnostics in changed files
• cargo clippy ... -- -D warnings is currently blocked by two pre-existing clippy::nonminimal_bool diagnostics in crates/base/src/calendar.rs:131 under Rust 1.95

AI assistance

AI was used to inspect the component and dock rendering paths, draft the implementation and tests, and prepare this PR description. I reviewed the resulting API and diff and ran the test and formatting checks listed above.

[huacnlee]

NOTE: This was completely written by AI but the issue is real. I was not able to find a way to set the height of the tab bar in any way.

Don't worry about this, we have totally accept AI written codes, even 100%.

[huacnlee]

Thanks for the report — the underlying problem is real, dock tab bars genuinely have no height escape hatch today. But I don't want to merge this API shape, and I think the diagnosis stops one level short of the actual cause.

1. tab_size does not name a size, it names a height

Size::Size(Pixels) has no single meaning in this library. Depending on the component it is:

• a square edge — StyleSized::size_with → .size(px), Icon, Avatar, OtpInput, Button's icon box
• a width — NumberInput → this.min_w(size) (crates/component/src/input/number_input.rs:150)
• a row height — Size::table_row_height
• a font size — input_text_size → size * 0.875

This PR adds a fifth meaning ("dock tab outer height") behind a public setter called set_tab_size, and the only way a caller learns which one they got is by reading the impl. If the dock needs to size its chrome, the API should name the thing it sizes: tab_bar_height() -> Pixels / set_tab_bar_height(Pixels). A Pixels in, a Pixels out, no enum to disambiguate.

2. The value silently becomes widths

The number passed as a height is then used for horizontal spacing in four places:

• crates/component/src/tab/tab.rs:90 — inner_paddings: height * 0.375
• crates/component/src/tab/tab_bar.rs:352 — default_gap: height * 0.375
• crates/component/src/tab/tab_bar.rs:372 — segmented padding_x: height * 0.125
• crates/component/src/tab/tab_bar.rs:388 — underline gap: height * 0.5

So "tab size" is a height in one place and a width in four, and the ratios do not reproduce the scale they claim to generalize. Checking them against the built-in steps:

	ratio gives	actual
default_gap @ Large (36)	13.5	16
inner_paddings @ Small (24)	9	10
inner_paddings @ Large (36)	13.5	16
underline gap @ Medium (36)	18	16
underline gap @ Small (30)	15	12
underline gap @ Large (44)	22	20

Size::Size(px(36.)) is therefore not Size::Large, and only Size::Size(px(32.)) happens to land on Size::Medium for the default variant. A custom value lands between the named steps by a rule no caller can predict, so these constants are unreviewable — there is nothing to check them against.

3. The pass is partial, which confirms it is a height and not a size

Size::Size reaches the outer height and the horizontal padding, but falls through to the Medium arm for inner_margins (tab.rs:108), radius / inner_radius / tab_bar_radius (tab.rs:351-386), the label font (_ => this.text_sm()), and the icon (_ => this.size_4()). A 44px tab is a medium tab with air around it. That may well be the right behaviour — but then the parameter is a height, and calling it a size promises scaling it does not deliver.

4. The motivation points at a different bug

This makes dock tabs clip when an application increases its global text scale.

Labels are rem-based (text_sm → rems(0.875)), tab heights are literal px(). That mismatch is the bug: the box does not follow the text. The library already solves exactly this in Icon, which derives its size from window.rem_size() (crates/component/src/icon.rs:150).

Making the dock chrome's fixed heights rem-derived fixes the clipping for every application without new public API, and without asking apps to recompute a pixel height each time they change scale — which is what this PR leaves them doing. I'd rather see that change. If an explicit per-app override is still wanted afterwards, it can be added on top, as set_tab_bar_height(Pixels).

5. A third height table in render_title

crates/component/src/dock/tab_panel.rs:373-378 introduces its own 20/24/36/30 mapping, while TabVariant::height maps 20/24/36/32. Medium disagrees, so set_tab_size(px(32.)) — the value that equals today's default tab bar — silently makes the single-panel title bar 2px taller than leaving the default alone. Whatever the final shape, this height needs to come from one place.

6. A closed bottom dock still clips

crates/base/src/dock/dock_area.rs:1932:

/// A closed bottom dock keeps this much, so its tab bar stays clickable.
pub const CLOSED_BOTTOM_STRIP: Pixels = px(29.);

With set_tab_size(px(44.)) that strip cuts off the tab bar it exists to preserve. Any height knob has to reach this constant too — another argument for fixing the scaling at the source rather than threading a value through the skin.

7. Docs and story

DockSkin's settings are documented in website/docs/dock.md and website/zh-CN/docs/dock.md; new public API has to land in both locales. There is also no story coverage for the new setting.

Minor

tab_size_changes_the_height_left_for_panel_content asserts medium_content - custom_content == px(12.), which encodes "the default bar is 32px" as an unexplained 12. Assert the two heights instead so a change to the default fails with a readable message.

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Requesting changes. Happy to take a PR that makes the dock's fixed chrome heights follow rem_size; if we still want an explicit override after that, let's land it as set_tab_bar_height(Pixels) rather than overloading Size.