GTK chrome: close the visual gap with VS Code — type size, vertical rhythm, tab/breadcrumb metrics

[JDonaghy]

Goal

Close the visual gap between vimcode's GTK chrome and VS Code. Side-by-side screenshot
(vimcode top, VS Code bottom, same screen/DPI, same file open) shows the difference is
almost entirely type size and vertical rhythm in the chrome, not colour and not
layout structure. Most of it is a handful of metric constants, so the payoff-to-risk
ratio is high.

This is an audit + parent issue. It is deliberately not one worker's dispatch —
see "Splitting" at the bottom.

What the screenshot shows

Reading top (vimcode) against bottom (VS Code):

• Everything in vimcode's chrome is roughly 15–25% larger. The menu bar, tab
  labels, breadcrumb and explorer rows all sit on a bigger grid. VS Code fits ~13
  explorer rows in the vertical space vimcode uses for ~7.
• The breadcrumb is as large and as bright as the code itself. In VS Code it is
  visibly smaller and dimmer than the editor text, and carries a file icon on the
  leaf segment.
• Tabs carry ordinal prefixes (1: main.rs, 2: mod.rs) and no file-type icon.
  VS Code shows a coloured language icon + bare filename, and marks the active tab
  with a top accent line.
• Menu mnemonics are underlined permanently (F̲ile E̲dit V̲iew). VS Code only
  reveals them while Alt is held.
• No indent guides in the editor; VS Code draws them by default.
• No panel separator borders. VS Code Dark Modern puts a 1px #2B2B2B line
  between activity bar / sidebar / editor; vimcode's panes butt together at the same
  background value, so the sidebar reads as part of the editor.
• Line numbers are uniformly bright. VS Code dims them (#858585) and brightens
  only the cursor's line.
• Activity-bar icons read slightly oversized against VS Code's.

The fixes, in priority order

Tier 1 — metric constants (highest payoff, smallest diff, vimcode-side only)

#	Change	Now	Target	Where
1	Explorer row height	28.0 px	22 px (VS Code list.rowHeight)	src/gtk/mod.rs:1572 (explorer_row_height_cell)
2	Decouple tab-bar height from the editor font	ceil(line_height * 1.6)	35 px fixed, independent of font_size	src/gtk/mod.rs:1416, and the three duplicate sites at :2647, :10207, :10346
3	Decouple the breadcrumb row from the editor font	+ line_height when settings.breadcrumbs	22 px row at ~12px text, dimmed fg	same sites as #2
4	Drop the ordinal prefix from tab labels	" {i+1}: {name} "	"{name}"	src/render.rs:10017 (+ the two [No Name] arms at :10022, :10025)
5	Stop underlining menu mnemonics unconditionally	every item gets &	pass & only while Alt is held, or drop it	src/render.rs:3265 (format!("&{name}"))
6	Turn indent guides on by default	setting exists, evidently off	default on	src/core/settings.rs (the indent_guides default)

Items 2/3/4 travel together — they are what makes the top of the window feel oversized.
Note item 4 has a hit-testing consequence: quadraui::TabItem's doc says backends locate
the filename "after the last \": \"" to underline it, so removing the prefix needs the
tab-bar underline/hit path re-checked, not just the format string.

Tier 2 — type stack and colour tokens (vimcode-side)

#	Change	Now	Target	Where
7	UI font stack	"Segoe UI, Ubuntu, Droid Sans, Sans" @ ui_font_size 10pt	put the real system UI font first — system-ui/Cantarell/Ubuntu — before falling through to generic Sans. On this Linux box neither Segoe UI nor Droid Sans resolves, so it lands on DejaVu Sans, which is wider and has a taller x-height than VS Code's result at the same nominal size	src/gtk/mod.rs:64 (UI_FONT_FAMILY), default in src/core/settings.rs:728
8	Line-number foreground	uniform	dim inactive (#858585), bright on the cursor line	theme tokens
9	Panel separator borders	none	1px #2B2B2B between activity bar / sidebar / editor	theme + GTK paint
10	Breadcrumb foreground	~code brightness	dimmed, distinct from editor text	theme tokens

Item 7 is a judgement call about which family, not a mechanical swap — worth eyeballing
before/after rather than trusting the constant.

Tier 3 — needs a quadraui change first (see the seam note)

#	Change	Why it's quadraui-side
11	Per-tab file-type icon	quadraui::TabItem has no icon field — quadraui/src/primitives/tab_bar.rs:114-132 is label / is_active / is_dirty / is_preview / is_closable only
12	Active-tab top accent line	the GTK tab-bar rasteriser distinguishes active by bg/fg only (quadraui/src/gtk/tab_bar.rs:177,187,197)
13	Activity-bar icon size	hardcoded "Symbols Nerd Font, monospace 20" ≈ 26.7px vs VS Code's 24px codicons — quadraui/src/gtk/activity_bar.rs:72. The 48px row itself (:22) already matches VS Code, so only the glyph size is off

Seam note — do not edit the pin

Cargo.toml:62 pins quadraui to a git rev
(29b98d6647ed808236dc3a11e97cd5d070192e91). Tier 3 therefore cannot land from a vimcode
branch: it needs a quadraui PR merged, released, and the pin bumped deliberately.
Do not edit that rev as a shortcut. Tiers 1 and 2 are entirely vimcode-side and can
ship without touching quadraui at all — which is the argument for doing them first and
measuring what gap is actually left.

Acceptance

Per this repo's black-box rule, assert on rendered output via GtkDriver
(src/gtk/testing.rs) — it paints into in-memory Cairo surfaces, so metrics are directly
assertable:

• explorer row pitch == 22 px
• tab-bar height stays 35 px across at least two different settings.font_size values
  (this is the assertion that actually pins the decoupling in v0.3.0: TUI-only binary (vcd) #2/v0.3.1: IDE polish, auto-pairs, build fixes #3 — a single-font-size
  test passes with the bug reinstated)
• breadcrumb row height and text size independent of font_size
• a tab label renders as main.rs, not 1: main.rs
• menu-bar items render with no underline when Alt is not held

A screenshot comparison is not a test — it is the reason for the issue, not the evidence
that it is fixed.

Out of scope

VS Code features visible in the screenshot that are not styling and should not be
smuggled in here: the "Open Editors" sidebar section, the inline git-blame annotation,
the minimap, the command-center pill in the title bar, and the editor's syntax-token
palette (a separate theme-fidelity question).

Splitting

Tier 1 is six independent one-liner-ish changes over two files and should be a single
dispatch. Tier 2 is a second. Tier 3 is a quadraui issue plus a pin bump, and should not
be queued until Tiers 1–2 have shipped and the remaining gap has been re-photographed.

[JDonaghy]

Split into three dispatchable children:

• Tier 1 — vimcode#700: chrome metrics (explorer row 28→22, tab/breadcrumb height decoupled from the editor font, tab ordinals, menu mnemonics, indent-guide default). Entirely vimcode-side. Queued.
• Tier 2 — vimcode#701: UI font stack, line-number/breadcrumb dimming, panel separator borders. Sequenced after Tier 1 — the font-family choice is a judgement call that should be made against corrected metrics, and it overlaps Tier 1's files.
• Tier 3 — quadraui#620: TabItem icon field, active-tab top accent, activity-bar glyph size. Filed in quadraui because none of it can land from a vimcode branch — Cargo.toml:62 pins quadraui by git rev, so this needs a quadraui PR plus a deliberate pin bump here afterwards.

This issue stays open as the audit/parent.

[JDonaghy]

Tier 1 follow-up: the three quadraui gaps are now filed

#700's review approved with the caveat that three of its six items were escalations,
not fixes, and that quadraui issues had to be filed before those acceptance bullets
count. They are:

#700 item	Status after Tier 1	quadraui issue
1 — explorer row 28→22px	not fixed — edit landed in dead code (explorer_row_height_cell is read only by the unreachable Msg::ExplorerClick path); live pitch is still line_height * 1.4	JDonaghy/quadraui#623
3 — breadcrumb row	half fixed — row height is a fixed 22px, but text still renders at editor font size	JDonaghy/quadraui#624
5 — menu mnemonics	not fixed — dropping & relocates the underline to char 0 rather than removing it (alt_char_byte_range never returns None for a non-empty label)	JDonaghy/quadraui#625

Items 2 (tab row pinned at 35px), 4 (no ordinal prefix) and 6 (indent guides default
on) did land and are visible.

Two things that block Tier 2 (#701)

1. VS Code parity (Tier 2a of #699): line-number and breadcrumb dimming #701 item 1 (UI font stack) is blocked on quadraui#624. vimcode's
   UI_FONT_FAMILY is fed to GtkBackend::set_ui_font, which is read by exactly two
   primitives — draw_dialog and draw_rich_text_popup. The tab bar, breadcrumb,
   file tree, status bar and menu bar all paint with the shared editor Pango layout.
   Changing the family today would be invisible on every surface this epic is about —
   the same shape of non-fix as VS Code parity (Tier 1 of #699): GTK chrome metrics — explorer row, tab/breadcrumb height, tab ordinals, mnemonics, indent guides #700 item 1. Item 1 should be deferred until
   quadraui#624 lands. Items 2–4 are theme-colour work and are not blocked.

2. Tier 3 (quadraui#620) is already merged in quadraui but invisible here —
   vimcode is pinned to 29b98d6, which is 6 commits behind quadraui's develop:

   dcc8721 feat(quadraui): TabItem icon, active-tab top accent, activity-bar glyph size (#620)
   251fb0f fix(quadraui): carry tab icons in a sidecar, not a TabItem field (#620)
   a2da705 fix(quadraui#620): keep active_accent: None meaning "no accent" on GTK
   17afbb4 fix(quadraui#620): don't add a Theme field — coord-tui builds Theme exhaustively
   

   Tab icons, the active-tab top accent and the activity-bar glyph size are done and
   waiting on a pin bump in Cargo.toml:62 + :124. That is probably the single
   highest visual return available right now.

[JDonaghy]

Pin bumped — Tier 3 is now partly live

develop @ 4356a0e moves the quadraui pin 29b98d66 → 17afbb42, picking up
quadraui#620. Two of Tier 3's three items went live with no vimcode change:

• Active-tab top accent — render.rs:6875/:7002 already passed
  active_accent: Some(theme.tab_active_accent) for the focused group; GTK previously
  ignored the field and now paints a 1px line at the active tab's top edge.
• Activity-bar glyph size — 20pt → 18pt against the unchanged 48px row.

Tab icons did not — quadraui deliberately made them opt-in via a sidecar slice
(Backend::draw_tab_bar_icons) rather than a breaking TabItem::icon field, so
adoption is vimcode-side work. Filed as #703.

Current state of #699

Tier	Status
Tier 1 (#700)	closed — 3 of 6 items real, 3 escalated to quadraui#623/#624/#625
Tier 2 (#701)	open — item 1 blocked on quadraui#624; items 2–3 ready now
Tier 3 (quadraui#620)	merged + pinned; tab icons remain, tracked in #703