Phase B.5 — GTK chrome catch-up onto quadraui primitives

[JDonaghy]

Goal

Close the gap between TUI (~95% on quadraui chrome) and GTK (~65%). After this wave, every chrome surface that has a quadraui primitive should consume it on both backends, with bug fixes living in one place. This is the wave Phase B.5 in PLAN.md refers to.

This is an umbrella issue — each checkbox is a self-contained slice that ships as its own PR / Path-A landing.

In scope (chrome only — editor viewport explicitly deferred to B.5b / Phase A.9)

• GTK Tooltip migration — `draw_hover_popup`, `draw_editor_hover_popup`, `draw_signature_popup`, `draw_diff_peek_popup` → `quadraui::Tooltip` via `quadraui_gtk::draw_tooltip`. Biggest deduplication win (4 popups → 1 primitive consumer). Also lets us drop the per-popup geometry math in each.
• GTK Dialog migration — `draw_dialog_popup` (quit confirm, close-tab confirm, error dialogs) → `quadraui::Dialog` via `quadraui_gtk::draw_dialog`. TUI consumer landed in Session 328 `83974fe`.
• GTK ContextMenu migration — `draw_context_menu_popup` → `quadraui::ContextMenu`. TUI consumer landed in Session 328 `9a52fd7`.
• GTK menu-dropdown migration — `draw_menu_dropdown` (top menu bar dropdowns) → `quadraui::ContextMenu`. TUI consumer landed in Session 328 `c6c0718`. Also closes Menu dropdown doesn't highlight items on mouse hover #181 (no hover highlight) since the primitive resolves hover state.
• GTK Completions migration — `draw_completion_popup` → `quadraui::Completions`. TUI consumer landed in Session 328 `afa14d9`.
• GTK debug toolbar migration — `draw_debug_toolbar` → `quadraui::StatusBar` via `bar.resolve_click` for hit testing. TUI consumer landed in Session 328 `f84c3c2`.
• GTK breadcrumb bar migration — `draw_breadcrumb_bar` → `quadraui::StatusBar` (same pattern). TUI consumer landed in Session 328 `553b207`.
• GTK ext-panel scrollbar — render + drag — bundled with TUI ext panel: scrollbar not drawn even when content overflows viewport #200 (scrollbar not drawn) + the deferred GTK migration noted in GTK explorer/ext-panel scrollbar drag: migrate to quadraui::dispatch_mouse_drag (off-by-thumb-length) #204's commit. Two parts: `draw_ext_dyn_panel` learns to render the scrollbar; the existing dead drag handler at `gtk/mod.rs:3022` migrates to `dispatch_mouse_drag`.

Pre-existing issues this wave subsumes

• Menu dropdown doesn't highlight items on mouse hover #181 — menu dropdown hover. Solved when GTK adopts `ContextMenu` (the primitive already tracks hover).
• TUI ext panel: scrollbar not drawn even when content overflows viewport #200 — TUI ext-panel scrollbar not drawn. Bundled with the GTK ext-panel migration.

Pre-existing issues this wave does not address

• Status-bar / command-line messages aren't mouse-selectable (GTK can't; TUI has offset bug) #194 — status bar message selection (different concern: text selection within the status bar, not the bar's layout migration).
• LSP signature help popup never shows data #180 — LSP signature help shows no data (that's an engine bug — server response not reaching the popup; unrelated to migrating the popup's renderer).
• GTK explorer indent guide lines #38 — explorer indent guides (feature add, not a migration).
• GTK find/replace: drag-to-select and double-click not implemented #62 — GTK find/replace selection (find/replace already migrated; this is a separate selection feature).
• Unified sidebar rendering via ScreenLayout #133 — "Unified sidebar rendering via ScreenLayout" (proposes a different architectural axis; revisit after this wave to see whether it's still desirable).

Suggested order

Tooltip first (biggest dedup), then Dialog (smallest single-surface change), then ContextMenu / menu-dropdown together (same primitive, different consumers), then Completions, then debug toolbar + breadcrumb together (same StatusBar pattern), ext-panel last (bundled with #200 render fix).

Rough size estimates

Slice	LOC delta	Risk
Tooltip	-200 / +50	Low — primitive proven on TUI
Dialog	-120 / +40	Low
ContextMenu + menu-dropdown	-200 / +60	Med — focus/hover handling
Completions	-150 / +50	Low
Debug toolbar	-80 / +30	Low
Breadcrumb bar	-60 / +25	Low
Ext-panel render+drag	-100 / +120	Med — currently no scrollbar at all

Cumulative: ~-900 LOC GTK draw.rs, +375 LOC primitive consumers.

Why now

• Phase B.4 (TUI chrome + cross-backend event routing) is substantially done.
• The quadraui north-star — "developer can write a cross-platform app on quadraui without knowing GTK / crossterm / Cocoa" — is currently true for picker-shaped surfaces but false for dialog/tooltip/completion-shaped surfaces because GTK still draws those bespoke. Closing the gap makes the north-star defensible across both backends in vimcode and unblocks the Postman-class validation app (Postman-class app as quadraui validation consumer #169).
• Bug fixes today (e.g. today's LSP status indicator no longer shows '…' while server is initializing #195 LSP indicator landing in core only) compound when both backends consume the same primitive. Every duplicated GTK draw function is a bug-fix that has to ship twice.

[JDonaghy]

Slice 1/8 landed: e1e76cd — draw_hover_popup migrated to quadraui::Tooltip. New quadraui_gtk::draw_tooltip rasteriser is feature-complete for all 4 popup migrations on this wave (handles plain text + styled_lines).

[JDonaghy]

Slice 2/8 landed: aaa9a3c — draw_signature_popup migrated to quadraui::Tooltip{styled_lines}. Live smoke-test deferred to after #180 fix (popup rendering depends on engine populating lsp_signature_help, which is broken pre-existing). Migration verified by code review.

[JDonaghy]

Slice 2/8 live-verified by user after #180 fix landed (ead8b56). Active parameter highlighting is correct (keyword colour). One follow-up: TUI vs GTK Tooltip borders look different — filed as #206 for the primitive-level fix.

[JDonaghy]

Slice 3/8 landed: e6650fa — draw_diff_peek_popup migrated to quadraui::Tooltip{styled_lines}. Live-verified in GTK: per-line +/- colour correct, action bar renders, placement falls back above when no room below.