arca-gui migrates from egui to GPUI and redesigns the window with GPUI Kit

[Villoh]

Migrate arca-gui from egui/eframe to GPUI and redesign the GPUI surface on top of
GPUI Kit. This covers phases 1 through 7 of the plan
added in docs/plans/migration-to-gpui.md.

Intentionally a draft: the GPUI window is already standing and looks good,
but there are still things to polish and platform validation to complete. The
list is below, under “What remains”.

cargo run -p arca-gui still launches the egui window. The GPUI window is
opt-in: cargo run -p arca-gui --features gpui. None of this changes what
users get yet.

---

Why

arca-gui was a ~4,600-line window in src/main.rs with business logic and
egui rendering interleaved. The goal is not just to change toolkits, but to
keep state out of the type that the toolkit renders.

What changes, layer by layer

1. State is separated from the toolkit

An AppController now owns the inputs, current folder, history, selection,
filter, sort order, jobs, and pending dialogs. It is driven through
AppAction instead of egui::Context.

The workers and message channel are unchanged: no disk or compression
operation touches the UI thread. That is precisely what allows two possible
windows to share the same logic, and what makes this PR incremental rather
than a big bang.

2. GPUI comes from GPUI Kit, not a Zed revision

The original plan pinned zed-industries/zed@3384317. That has been removed:

• gpui-component is built against gpui-pre ^0.3.
• Keeping the git revision left two copies of GPUI in the dependency graph,
  which do not link together.

The solution is to rename the packages in Cargo.toml, so not a single
use in the code changes:

gpui = { package = "gpui-pre", version = "0.3", default-features = false, optional = true }
gpui-component = { version = "0.6", optional = true }
gpui-kit-assets = { version = "0.6", optional = true }

Resulting unified graph: gpui-pre 0.3.4 + gpui-base 0.6.0 +
gpui-component 0.6.0. Reproducibility comes from Cargo.lock, which is no
longer in .gitignore — a pin that is not versioned is not a pin.

spikes/gpui keeps the original revision: it records what was validated in
G1 and is not rewritten retroactively.

3. Monochrome light and dark theme (arca-gui/src/gpui_theme.rs)

Six grays per mode, hue 225 (the night blue from brand/BRAND.md) at 8–12%
saturation, inverted between light and dark.

	Dark	Light
background	#0E1014	#FAFAFB
surface	#14161B	#FFFFFF
raised	#1B1E25	#F3F4F6
border	#262A33	#E3E5EA
text	#E4E6EB	#14161B
muted	#8B909C	#666B76

There is no accent color. The selected row, keyboard cursor, and focus
ring use the text color at different strengths (12%, 100%, 70%). Contrast is
guaranteed by construction rather than by a table someone has to maintain: if
the text is readable, the selection is visible.

The only exceptions are danger and warning. They are not decoration — they
distinguish “extracted” from “not extracted”, and anyone scanning before
reading needs to be able to see that distinction. They remain at the lowest
chroma that is still legible as red and amber on both backgrounds.

The tokens are the ones from gpui-component; Arca does not add its own
token layer, it only specifies the value of each token.

Radius 4/6 px, row height 26 px, base font size 13 px.

4. Layout redesign

The reference is Nohrs (the same problem:
a file explorer) with the density of DBFlux.

Before: a stack of floating strips in p_3 + gap_3. Now: full-bleed regions
separated by 1 px lines, with padding inside each bar so every divider spans
the full width.

┌─ actions (40 px) ────────────────────────── [filter] ──┐
├─ ← → ↑ │ path (34 px) ─────────────────────────────────┤
│ folders  │  full-bleed table                           │
│ (224 px) │                                              │
├──────────┴──────────────────────────────────────────────┤
│ file summary                 N visible │ M selected      │
└─────────────────────────────────────────────────────────┘

• Sidebar with the folder tree (gpui_component::sidebar). It is content,
  not chrome: until now, reaching a deeply nested file meant double-clicking
  down and navigating back up. The current folder branch opens automatically;
  the rest stays closed. The tree is cached by (file path, entry count) —
  rebuilding it in render meant walking all names 60 times per second during
  extraction.
• Status bar attached to the bottom (gpui_component::status_bar), with
  the summary on the left and counters on the right. The counters used to live
  in the middle of the navigation row, pushing the breadcrumbs against the
  right edge.
• Borderless buttons, with the background appearing only under the pointer.
  Seven outlined boxes read as seven competing things.
• Icon arrows (IconName, via gpui-kit-assets) instead of ‹ › ↑.
• Floating menus: the overflow menu used to be rendered in flow and pushed
  half the window down.
• Full-bleed table, with no border or radius of its own, a fixed header,
  and alternating stripes.

5. Accessibility

What was already there is preserved and not downgraded: AccessKit roles,
visible focus, keyboard navigation, focus trapping in modals, and the
background subtree without roles while an overlay has focus. Icon buttons keep
their accessible names as words.

---

Verification

• cargo test --workspace — green (default build, egui).
• cargo test -p arca-gui --features gpui — 38/38.
• cargo fmt -p arca-gui -- --check — clean.
• Visual inspection on Windows with a nested test ZIP: folder tree, navigation,
  table, status bar, dark theme.

Two new tests in gpui_theme.rs:

• WCAG contrast for every ink color on each of the three backgrounds, in
  both modes. With no accent, the palette is the accessibility, so this is
  the test that matters.
• Distinct, opposite palettes: two grays that are the same gray, or a mode
  that stops being the inverse of the other. These are copy-and-paste slips that
  compile and do not show up in a diff of thirty hex literals.

I wrote a third test that failed (it assumed the three backgrounds are ordered
in the same direction in both modes). That is false: in light mode a panel is
raised toward white and a hover is tinted away from it. Rather than tuning
the threshold until it passed, I replaced it with the thing that can actually
break.

---

What remains

That is why this is a draft.

1. Internal widgets are still hand-rolled. They need to be replaced with
   the ones from gpui-component:
   • Input — removes the roughly 400 lines of FilterInput and its UTF-16
     IME contract. This is the largest pending deletion.
   • Modal/Root, Popover, Table, Notification.
2. Menus float with absolute and fixed offsets (top(38.) right(232.)),
   rather than being anchored to their trigger. They break if the filter width
   or font size changes. The correct solution is Popover.
3. There is no settings dialog in the GPUI surface, which would allow the
   theme and language to be changed without editing gui.conf. For now the
   preference is read at startup and System follows the desktop.
4. Incomplete platform matrix. Only Windows GNU has been validated. Linux
   and macOS are still unmarked in docs/plans/gpui-spike-baseline.md, and
   they should not be marked without a real test.
5. Screen reader not yet validated with Narrator/NVDA on the GPUI window.
   The plan says any accessibility limitation blocks removing egui, so this is
   required before phase 8.
6. egui is still included. Phase 8 (removing eframe, egui,
   egui_extras) is not done, and should not be done until 4 and 5 are closed.

Notes for reviewers

• Three button labels were reconstructed. During the work I corrupted 15
  multiline dialog_button calls with a bad regex. The on_click bodies
  survived intact, and modal_focus_targets/modal_enter pin the exact id and
  focus handle of each button, so the reconstruction is deterministic except
  for the visible text of three: "Set Password"/"Unlock",
  "Keep Both"/"Keep Both Always", and "Delete". They deserve a look.
• Commit 1 contains the spike and the plan; commit 2 contains the code. They
  are not split further because any intermediate cut inside gpui_shell.rs
  produces a commit that does not compile.
• Settings::load already reads the columns key, which was a parity defect
  noted in G1. The gui.conf format does not change.