navigation: window buttons, window drag and wheel tab switching on Linux and Windows

[daniarjabagin]

Closes #1916

What

Under window.decorations = "Disabled" the rio-rendered tab strip is the only chrome a Linux or Windows window has, but outside macOS it offered none of a title bar's controls. This adds three opt-in [navigation] keys:

[window]
decorations = "Disabled"

[navigation]
window-buttons = true   # ⎯ ☐ ✕ at the right end of the strip
drag-window = true      # left-drag on empty strip area moves the window
scroll-tabs = true      # mouse wheel over the strip switches tabs

All three default to false. macOS ignores window-buttons (the traffic lights already occupy the strip) and keeps allowing the drag, exactly as before.

How

window-buttons (renderer/island.rs)

• New WindowButton enum and geometry helpers (window_buttons_width, window_button_hit). Buttons are 36 logical px each, anchored to the right edge — the same anchoring the tab close × uses.
• tab_strip_layout gains a reserved_right argument; slots and the lone-tab title budget / centring subtract it, so tabs shrink and a floating (dragged) tab never covers the buttons.
• Drawn with sugarloaf.line / rounded_rect like the existing close ×. Hover backdrop comes from island_fills so it adapts to light and dark backgrounds; close hovers red with a white glyph.
• Click routing (screen/mod.rs::handle_island_click): the button hit test runs before the tab-slot logic, because the buttons live past the last slot where a press would otherwise start a window drag. The pressed button is stored as pending_window_button and consumed in application.rs, which owns the router / event loop a close needs. Close follows the CloseRequested contract: confirm_quit() when confirm-before-quit is set, otherwise remove the route and exit once none remain.
• Hover is tracked next to the close-× hover in update_close_button_hover (both are evaluated so neither update is skipped).

drag-window (screen/mod.rs)

• start_window_drag loses its macOS cfg; on_chrome_press calls it when navigation.allows_window_drag() — always on macOS, opt-in elsewhere. drag_window() is winit's xdg_toplevel.move / _NET_WM_MOVERESIZE / WM_NCLBUTTONDOWN, so no platform code is added.
• Double-click → toggle maximize was already cross-platform and is unchanged.

scroll-tabs (application.rs, screen/mod.rs)

• MouseWheel over the visible strip goes to Screen::scroll_tabs_over_strip before terminal scrolling. Line deltas switch per notch; pixel deltas (touchpads) accumulate, one tab per WHEEL_PX_PER_TAB (40 logical px). Switching reuses the same switch_to_next/prev + switch_context_visibility sequence as the palette actions.

Config (rio-backend/src/config/navigation.rs)

• Three bool fields with serde defaults; draws_window_buttons() / allows_window_drag() encode the platform rules in one place.

Tests

• tab_strip_layout_reserves_window_buttons — slots shrink by the reserved width, tab region ends before the buttons, negative reservation is ignored.
• window_button_hit_maps_thirds_left_to_right — hit test per button, right margin and disabled state.
• single_title_stays_clear_of_window_buttons — lone-tab title never runs under the buttons; budget shrinks by the reservation.
• Existing layout / title tests updated for the new argument.

Locally: cargo fmt -- --check, cargo clippy --all-targets --all-features (no warnings in touched files), cargo test --features wgpu (all green).

Tested on

• Linux · Wayland (GNOME 50, Vulkan backend): buttons, hover, minimize / maximize / close, drag, double-click maximize, wheel switching, tab drag-reorder and close × unaffected, single-tab title clear of the buttons.
• Linux · X11 (XWayland): same as above.
• Windows: builds in CI (msys2-build-test, windows-latest); drag_window() / set_minimized / set_maximized are winit's existing Windows paths. Not hand-tested — happy to adjust if someone can try it.
• macOS: behaviour unchanged (window-buttons is a no-op, drag stays always-on); verified by reading the cfg paths, no hardware to test.

Docs

Companion PR to rioterm.com: raphamorim/rioterm.com#3

controls.mp4