UI system: SDL2 panels, menu, keyboard, and download browser

The coopdx-rs launcher renders everything through SDL2 — there is no native widget toolkit. All UI elements are drawn each frame using textured quads, software-rasterised rounded rectangles, and pre-rendered TTF glyphs cached as SDL2 textures. The src/ui/ directory contains nine modules that together form the complete UI layer.

UI module overview

common

Shared types and rendering helpers: UiItem, ItemType, draw_item_selector, ensure_selection_visible. Used by every sub-screen that renders a scrollable item list.

panel

PanelState — the sliding side panel that hosts all sub-screens. Handles smooth slide_x animation, rounded-corner background rendering, header/footer caching, and SubScreenType dispatch.

splash

SplashState — 7-phase loading progress bar shown during startup asset loading. Manages fade-in, progress animation, and the transition signal to the main game screen.

vinyl

Animated spinning vinyl disc rendered in the bottom-right corner of the main screen. Rotates at VINYL_ROT_SPEED = 90°/s and dims when a sub-screen is open.

logo

Game logo rendering with a drop shadow offset (SHADOW_OFFSET = 8px). The logo is scaled to fit within MAX_LOGO_W × MAX_LOGO_H = 900 × 450 pixels while preserving aspect ratio.

keyboard

VirtualKeyboard — on-screen keyboard overlay for text input in SDL2. Used whenever a text field is activated (network fields, profile names). Characters are appended to a caller-supplied buffer.

network_form

NetworkFormState — renders the network configuration form. Shows different fields depending on the active NetworkMode: no extra fields for Local, IP+port for Client, port for Server, password for CoopNet.

download_browser

DownloadBrowserState, DlFocusMode — the full download browser UI including search bar, tag chips, author dropdown, paginated mod list, and download progress overlay.

SubScreenType enum

The panel system routes keyboard input and rendering to the correct sub-screen using the SubScreenType enum defined in src/ui/panel.rs:

pub enum SubScreenType {
    ModManager,
    DynosPacks,
    Network,
    DownloadBrowser,
    Profiles,
    ProfileDetail,
}

Each variant maps to a header label displayed at the top of the panel:

Variant	Header Label
`ModManager`	`MOD MANAGER`
`DynosPacks`	`DYNOS PACKS`
`Network`	`NETWORK`
`DownloadBrowser`	`DOWNLOAD MODS`
`Profiles`	`PROFILES`
`ProfileDetail`	`PROFILE`

The mapping from arc-menu button index to sub-screen is handled by subscreen_for_menu_index(idx: usize) -> Option<SubScreenType>. Indices 0–4 map to ModManager, DynosPacks, Network, DownloadBrowser, and Profiles respectively.

Panel system

PanelState in src/ui/panel.rs drives all sub-screens. It maintains a continuous slide_x value that eases toward either a visible or a hidden target_x position, giving the panel a smooth slide-in/slide-out animation.

pub struct PanelState {
    pub slide_x: f64,
    pub active: Option<SubScreenType>,
    pub header_extra: Option<String>,
    pub footer_hint: Option<String>,
    // … cached SDL2 textures for header and footer text
}

Key constants and ratios:

Constant	Value	Meaning
`PANEL_W_RATIO`	`0.62`	Panel width as a fraction of window width
`PANEL_H_RATIO`	`0.72`	Panel height as a fraction of window height
`SLIDE_SPEED`	`8.0`	Exponential easing multiplier per second
`HEADER_H`	`48 px`	Height of the header band
`FOOTER_H`	`36 px`	Height of the footer hint band
`CORNER_RADIUS`	`16 px`	Rounded corner radius

Rounded corners

The panel background is rendered without any native rounded-rectangle primitive (no SDL2_gfx dependency). Instead, make_rounded_bg pre-renders an ARGB8888 surface, fills it with PANEL_BG (rgba(10, 5, 30, 230)), and sets corner pixels outside the radius to fully transparent using a midpoint-circle test. The resulting texture is cached in PanelState and only regenerated on window resize.

Opening and closing

impl PanelState {
    pub fn open(&mut self, sub_type: SubScreenType, win_w: u32)
    pub fn close(&mut self, win_w: u32)
    pub fn update(&mut self, dt: f64, win_w: u32)
    pub fn is_visible(&self, win_w: u32) -> bool
}

open sets active and moves target_x to the panel’s centred visible position. close moves target_x to win_w (fully off-screen). update advances slide_x each frame using the formula slide_x += (target_x - slide_x) * (SLIDE_SPEED * dt).min(1.0), and clears active once the panel is fully hidden. The panel body area (below the header, above the footer) is computed by panel_body_rect(win_w, win_h, slide_x) -> Rect. Sub-screens use this rect as their drawing region.

The arc menu in src/ui/menu.rs presents five buttons in a curved layout on the right side of the screen. Buttons are offset horizontally according to their distance from the centre item, creating the characteristic arc shape.

pub const MENU_ITEM_COUNT: usize = 5;

pub static MENU_ITEMS: [ArcMenuItem; MENU_ITEM_COUNT] = [
    ArcMenuItem { label: "Mod Manager",   icon: "*" },
    ArcMenuItem { label: "DynOS Packs",   icon: "#" },
    ArcMenuItem { label: "Network",       icon: "~" },
    ArcMenuItem { label: "Download Mods", icon: "v" },
    ArcMenuItem { label: "Settings",      icon: "=" },
];

The menu supports both keyboard navigation (W/S or Up/Down to cycle, with wraparound) and mouse hit-testing via hit_test_menu_button(mouse_x, mouse_y, win_w, win_h) -> Option<usize>. The highlight_y float tracks the animated highlight bar position, which eases to the selected button’s Y coordinate each frame.

Virtual keyboard

In an SDL2 application there is no platform text-input widget, so the launcher ships its own on-screen keyboard in src/ui/keyboard.rs. The VirtualKeyboard is used whenever a text field is activated — network form fields (IP address, ports, CoopNet password) and profile name operations (create, rename, player name edit).

pub struct VirtualKeyboard { /* opaque */ }

impl VirtualKeyboard {
    pub fn new() -> Self
    pub fn open(&mut self)
    pub fn close(&mut self)
    pub fn move_up(&mut self)
    pub fn move_down(&mut self)
    pub fn move_left(&mut self)
    pub fn move_right(&mut self)
    pub fn confirm_if_active(&mut self, buffer: &mut String) -> bool
    pub fn active: bool  // field
}

The keyboard renders as an overlay on top of the current sub-screen. Arrow keys (or D-pad) navigate between keys. Enter/Space confirms the current character and appends it to the buffer. A second Enter on the confirmation key (typically OK / ↵) returns true from confirm_if_active, signalling the caller to commit the edit.

On desktop systems SDL2 delivers TextInput events for physical keyboard typing. The launcher’s event loop appends alphanumeric characters and permitted punctuation (. - _ @ space) directly to the edit buffer, so the virtual keyboard is mainly useful for Steam Deck / gamepad users.

UiItem and ItemType

The generic item selector used by Mod Manager, DynOS Packs, Profiles, and Profile Detail is built on two types from src/ui/common.rs:

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum ItemType {
    Toggle,  // checkmark (✓) or dash (—) icon; Enter/Space toggles enabled
    Text,    // shows a label and its current value; Enter opens the virtual keyboard
    Action,  // special synthetic item (e.g. "+ New Profile"); triggers an action
}

#[derive(Debug, Clone)]
pub struct UiItem {
    pub name: String,      // display label
    pub rel_path: String,  // filesystem key (mod filename, profile dir name, config key)
    pub enabled: bool,     // toggle state
    pub item_type: ItemType,
    pub value: String,     // current value for Text items
}

draw_item_selector renders a scrollable list of these items inside a panel body rectangle. It accepts pre-rendered SDL2 textures for both item names and the three icon variants (check ✓, dash —, plus +) and performs zero allocations per frame. Key layout constants from src/ui/common.rs:

Constant	Value	Meaning
`DEFAULT_VISIBLE_ROWS`	`9`	Visible rows in standard item lists
`DL_VISIBLE_ROWS`	`6`	Visible rows in the download browser mod list

Scroll state is managed by ensure_selection_visible(selected, scroll, visible_rows) -> usize, which adjusts the scroll offset just enough to keep the selected row on screen (no over-scroll).

Toggle items

Used for mods, DynOS packs, and most profile settings. The enabled flag drives the icon: ✓ when enabled, — when disabled. Pressing Enter or Space flips enabled and immediately calls the appropriate manager write function.

Text items

Used for the Player Name field in Profile Detail. Displays the current value string next to the label. Pressing Enter opens the VirtualKeyboard with value pre-populated in the buffer.

Download browser

DownloadBrowserState in src/ui/download_browser.rs is the most complex UI component. It combines a real-time search bar, tag chip filters, an author dropdown with autocomplete, a paginated mod list, and a download progress overlay.

Focus modes

pub enum DlFocusMode {
    ModList,     // Up/Down navigate the mod list
    TagChips,    // Left/Right navigate the tag filter chips
    AuthorInput, // Author dropdown / autocomplete
}

Tab (or the gamepad Start button) cycles through the three focus modes. The active focus mode determines which element receives arrow-key input.

Key state fields

pub struct DownloadBrowserState {
    pub db: Option<DbFile>,           // loaded mod database
    pub index: Option<SearchIndex>,   // pre-built search index
    pub sorted: Vec<usize>,           // default-sort order (by rating desc)
    pub search_text: String,          // current search query
    pub filtered: Option<Vec<usize>>, // filtered index into sorted, or None = show all
    pub selected: usize,              // selected row within current page
    pub scroll: usize,                // scroll offset within current page
    pub page: usize,                  // current page (DL_PAGE_SIZE = 15 items/page)
    pub active_tags: Vec<String>,     // active tag filter chips
    pub active_author: Option<String>,// active author filter
    pub focus_mode: DlFocusMode,
    pub top_tags: Vec<(String, usize)>, // top-9 tags by frequency for chip display
    // … author dropdown state, per-row texture cache
}

The database and search index are loaded lazily on the first time the sub-screen is opened. Filtering is re-computed whenever the search text, active tags, or active author changes using filter_mods_combined from the download manager.

Download progress overlay

While a download is in progress the browser renders a progress bar overlay on top of the mod list. The overlay reads DownloadProgress from the shared Arc<Mutex<DownloadProgress>> and displays bytes downloaded, percentage, and — after extraction — the number of mods extracted.

Building

Internals

UI system: SDL2 panels, menu, keyboard, and download browser

UI module overview

common

menu

panel

splash

vinyl

logo

keyboard

network_form

download_browser

SubScreenType enum

Panel system

Rounded corners

Opening and closing

Arc menu

Virtual keyboard

UiItem and ItemType

Toggle items

Text items

Download browser

Focus modes

Key state fields

Download progress overlay

Build docs developers (and LLMs) love

Building

Internals

Documentation Index

​UI module overview

common

menu

panel

splash

vinyl

logo

keyboard

network_form

download_browser

​SubScreenType enum

​Panel system

​Rounded corners

​Opening and closing

​Arc menu

​Virtual keyboard

​UiItem and ItemType

Toggle items

Text items

​Download browser

​Focus modes

​Key state fields

​Download progress overlay

Build docs developers (and LLMs) love

UI module overview

SubScreenType enum

Panel system

Rounded corners

Opening and closing

Arc menu

Virtual keyboard

UiItem and ItemType

Download browser

Focus modes

Key state fields

Download progress overlay