craft-libs/docs/ui-docking.md

Panel docking

craft_ui::docking lets the user arrange an app's panels: tab groups, docks with columns, splits, floating windows and iconic columns. craft-ui owns the layout model, the renderer and the gestures. The app owns panel contents, commands, document state and where layouts are saved.

Office apps that draw their own document with a few task panes beside it use craft_ui::panes instead.

Profiles

DockStyle::profile picks one of two arrangements. Both use the same model and actions.

Profile	Arrangement	Apps
Profile::Dock (default)	Docks on the window's start and end edges. A dock has columns; a column has stacked groups and can collapse to icons. Frames (tab groups and splits) fill the space between the docks.	Creative apps with a canvas and panel docks
Profile::Frame	A split tree of tab frames. Frame menus offer Maximize Frame, Undock and Close Other Panels. Ctrl/Cmd + tab drag undocks. Drops on all four outer edges split at the root.	Apps whose whole window is frames

Sides are logical: Side::Start is the left edge, or the right edge when DockStyle::right_to_left is set. egui has no app-wide direction, so the app sets it from its locale. Right to left, tab strips also mirror: the first tab is rightmost, the group header and drag bar are at the left, and drop insertion slots follow. Accordion headers and floating title bars (chrome::TitleBar, through ChromeStyle::right_to_left) mirror too. Frame splits keep their sides.

Model

Layout<P> holds an optional root Node<P> and a list of Floating windows.

Node	Holds
Tabs	A tab group: panels and the active index.
Split	Two children, horizontal or vertical, with a ratio or a fixed first or second size.
Stack	Accordion entries, each open or closed, with an optional height.
Dock	A side and its columns. A Column has Groups, a width, and iconic.

A Floating window has one or more stacked groups, a rect, and an optional native-window rect. Floating::single makes a window with one group; panels() and contains() read it.

P is the app's panel id: stable, not localized, Clone + Eq + Hash. The renderer also needs Debug + Send + Sync + 'static. A panel occurs at most once in a layout.

Change a layout only through Actions:

use craft_ui::docking::{Action, Floating, Layout, Node, Placement};

let mut layout = Layout {
    root: Some(Node::Tabs { panels: vec!["layers", "properties"], active: 0 }),
    floating: vec![Floating::single(vec!["brushes"], 0, [40.0, 80.0, 240.0, 300.0])],
};
layout.apply(Action::Move {
    panel: "brushes",
    anchor: "layers",
    placement: Placement::Tab { before: Some("properties") },
})?;
# Ok::<(), craft_ui::docking::DockError>(())

• Move, MovePanel, Float and Close act on one panel. MoveGroup, MoveColumn and MoveWindow move a group, a dock column or a floating window, named by any panel in it.
• A Target is At { anchor, placement }, Float { rect } or NewDock { side }.
• A Placement is a tab position, a split zone, a new group above or below (with an optional preferred height), a new column, or a new frame on an outer edge.
• Containers take different placements: a dock column takes groups and columns, not splits. Placement::for_zone(layout, anchor, zone, right_to_left) maps a left, right, top, bottom or centre zone onto what the anchor's container takes: a split in the frame tree, a new column on that side or a new group above or below in a dock, a group above or below in a floating window.
• Other actions select, minimize, collapse to icons, resize splits, columns and groups, and move a floating window to its own OS window (SetNative, feature native-windows).

Every action checks the whole result before it commits. A bad target, a duplicate id, a stale split path or invalid geometry returns a DockError and leaves the layout unchanged. The source panel is never lost. apply, apply_with_permissions and apply_all return the panels the actions closed, so the app can release their state; closed_by lists them before applying.

Permissions limit what may happen to a panel: movable, floatable, closable, accepts_tabs and accepts_splits. apply_with_permissions enforces them for every action, whoever issues it. Permissions::PROTECTED keeps a canvas docked and open and takes neither tabs nor side splits; a group dropped on it floats there. An app that opens panels beside its canvas uses Permissions { accepts_splits: true, ..Permissions::PROTECTED }.

Workspace

Workspace<P> is what an app saves: the layout, the hidden panels with where each one goes back, an optional saved arrangement for Reset, and locked.

Method	Does
apply, apply_all	Apply actions all or none, with the app's permissions. Closed panels are recorded in hidden. While locked, moving, floating and closing are refused.
set_visible(panel, visible, fallback)	Hide a panel, or show it at its recorded place, else beside fallback.
reconcile(registered, default)	Fit a loaded workspace to the panels the app has now. Unknown panels move to hidden, so they come back when registered again.
reset(default)	Return to the saved arrangement, or to default for a built-in workspace.
validate	Check a loaded workspace before use.

With the serde feature, Workspace, Layout and Action serialize. Files without a version are v1 and load unchanged. craft-ui does not choose where files go; the app does.

Rendering

DockArea::new(id).show_content(ui, &layout, &style, &mut content) draws a layout snapshot and returns a DockOutput. It does not change the layout. The app applies output.actions through its own command path, so pointer, keyboard and control commands share one path.

DockContent<P> is the app's adapter. It draws each panel body and gives its label, icon, permissions, size limits, preferred floating size, panel menu and group header. The group header sits at the end of the tab strip; group_header_width gets the strip width, so a header that draws its own tabs can size them to the room left. bare hides the tab strip of a lone panel. An icon is an image (icon), or drawn in code by paint_icon, which gets a painter, the icon rect and the tint color and returns true when it painted. Panel bodies keep a stable egui id when they move, so widget memory survives.

DockOutput also has:

• focused: the panel the user focused this frame: a clicked tab, a drop, or a press of any pointer button in a panel body, bare and protected panels included. Commands, raise_panel and workspace resets never set it; a host that tracks the last focused panel clears it when a reset or command hides, closes or moves that panel.
• events: requests outside the layout, such as DockEvent::ToggleMaximize (Frame) and DockEvent::ToggleAutoCollapse (Dock), and DockEvent::FlyoutChanged { panel } (Dock): the flyout opened, switched or closed (None). It comes once per change, whatever made it: an icon, Escape, a press outside, a raised panel, open_flyout or close_flyout.
• error: a rejected gesture.

Builder options on DockArea:

Option	Use
maximized(panel)	Maximize Frame: that frame fills the area.
hide_panels(HidePanels)	Hide panels for Tab and Shift+Tab. Protected panels stay.
auto_collapse_iconic(on)	Close a flyout on Escape or a click outside (default on).
raise_panel(panel)	Raise a panel's floating window, or open its flyout, this frame. It moves no keyboard focus and does not set DockOutput::focused.
blocked_rects(rects)	App overlays that docking gestures must ignore.

DockStyle::close_menu_items and DockStyle::close_button hide the built-in Close menu items and the floating window ×. They hide controls only: a panel that stays closable still closes by the app's own Close items and commands.

DockArea::chrome_has_focus tells whether keyboard focus is on docking chrome rather than on a panel body. Focus follows only the user's input: a tab keeps keyboard focus until a command or a workspace reset hides, closes or moves its panel, and then gives it up. begin_drag hands a drag that started in the app's own chrome to the area. docking::chrome has the floating frame, the title bar and drop_preview (the drop preview docking and craft_ui::panes both draw) for apps that draw their own windows.

Tab strips

Every shown tab carries its panel's full name. Tabs that do not fit go to the "…" button at the end of the strip, whose menu lists the hidden panels; choosing one makes it active. The active tab is always shown, in its logical place: when it lies past the tabs that fit, it takes the place of the last of them. Only an active tab wider than the room beside the "…" button, or a lone tab wider than the strip, is cut with "…", and its tooltip names it in full. The drag bar always keeps its reserved width: tabs go to the overflow menu before it narrows, and only a strip narrower than the bar itself shrinks it. A tab dropped on the "…" button goes to the end of the group.

Floating size

A panel or group floated by a drag, a menu or a command without a rect takes its content height: the title bar, the tab strip and the height the active panel's content used when it was last drawn (the content ui's min_rect), at least 120 points. The most it takes is the height of DockContent::floating_size, or 320 points for a panel leaving a dock or frame and a group leaving a dock. A short panel therefore floats as a short window. Content that reaches the bottom of its body (a panel that allocates its whole rect) has no height of its own: it floats at the floating_size height, else at 320 points. A panel's width is its floating_size width, else its docked width; a group keeps its docked width. A flyout fits its content the same way, without the title bar.

• A panel the area has not drawn floats at the floating_size height, else the docked height capped at 320, or at a FloatSpace::with_panel_size size for commands.
• A group from a frame without a floating_size keeps its frame size. A dock column of several groups floats at the tallest floating_size height, else its column height capped at 320.
• A rect from a command and a window the user resized keep their size; floating windows are not refitted while open.
• Docked layout does not change: a column's last group still fills the column.

Gestures

• A tab drags one panel. A group's drag bar (the space beside its tabs) or its top border drags the whole group. A column bar drags a column. A floating window's title bar drags the window.
• Drop previews show the destination: an outline for tabs, a line for a new group, column or dock, a filled half for a split. Escape or losing window focus cancels a drag.
• Double-click a tab or drag bar to minimize a group to its strip.
• In the Frame profile, the mouse wheel over a tab strip selects the next or previous tab, and a middle click on a closable panel's tab closes it. DockStyle::wheel_selects_tabs and middle_click_closes_tabs turn either on or off in any profile.
• A column's »/« button collapses it to icons, one icon per panel with a separator between groups. Widen an iconic column to show panel names. Click an icon to open its group as a flyout on the canvas side. Drag an icon to move its panel, or the grip above an icon group to move the group.

Commands

With the serde feature, DockCommand<P> is the shared control vocabulary. The app puts the command id under "command" in the params and deserializes them. craft-ui has no serde_json dependency.

let mut params = params.clone();
params["command"] = json!(id);
let command: DockCommand<PanelId> = serde_json::from_value(params)?;
if command.is_app_owned() {
    return app.workspace_command(command);
}
let space = float_space(ctx, DOCK_ID).unwrap_or_else(|| FloatSpace::new(ctx.content_rect()));
let closed = command.apply(&mut app.workspace, style.right_to_left, &space, |panel| app.permissions(panel))?;

Id	Params (? is optional)
window.panel.layout	{action}: a serialized Action
window.panel.move	{panel, anchor, zone?, before?, group?, placement?}
window.panel.float	{panel, x?, y?, width?, height?, group?, column?}
window.panel.dock	{panel, anchor?, zone?, before?, group?, column?, side?}
window.panel.activate, .close, .closeGroup, .closeOthers	{panel}
window.panel.minimize	{panel, minimized?}
window.panel.iconic	{panel, iconic?}
window.panel.window	{panel, native?}
window.workspace, .new, .delete	{name}
window.workspace.reset, ui.dock.reset	{}
window.workspace.lock	{locked?}
ui.dock	{action}, or {operation, …}

• zone is tab, center, left, right, top, bottom, above or below. before names the tab to insert before; null appends. left, right, top and bottom follow Placement::for_zone, so beside a docked panel they make a new column or group. apply and to_actions take the layout's right_to_left for that.
• group moves the panel's whole group; column moves its dock column, or its whole window when it floats. side is Start or End.
• window.panel.float without a rect floats what it moves at the size a drag floats it at (see Floating size). It goes inside the dock area, 24 points from its top-left corner, so it stays clear of the menu bar. The area records that geometry in a FloatSpace each frame; float_space(ctx, id) reads it and apply and to_actions take it. Before the area has drawn, FloatSpace::new(area) floats a panel at 260x320 and a group or column at its column width by 320. FloatSpace::with_panel_size(panel, size) gives a panel the area has not drawn (a hidden panel, a headless run) the host's own size; a size the area recorded wins. x, y, width and height replace the matching values, and a group or column that is already a whole floating window keeps its rect.
• Without a value, minimized, iconic and locked toggle.
• ui.dock operations are those of LightCraft, PDFCraft, CADCraft and SoundCraft: open, close, activate, float {rect}, moveFloating {rect}, move {target, zone?, before?}, setStackOpen {open}, resizeStack {height} and resizeSplit {path, size}. rect is [x, y, width, height].

to_actions returns the actions a command makes without applying them. apply applies them all or none and returns the panels it closed, so the app can release their state. Showing a hidden panel and Lock change the workspace directly. Named workspaces and Reset belong to the app: is_app_owned is true for them and apply refuses them. DockCommand::IDS lists every id for the app's command registry. App-specific aliases stay in the app.

Labels

Every visible and accessibility label is a *_label field of DockStyle, with an English default. DockStyle::localized sets them all from one keyed lookup:

let style = DockStyle::from_ui(ui).localized(|key| app.strings.get(key).cloned());

Keys are dock.close, dock.float, dock.collapseToIcons and so on; the rustdoc of localized lists them all. DockContent::label and action_label can shape panel and action text further; the logical label stays the accessibility name.

Testing

DockArea::hit_rects(ctx, id) lists every gesture source drawn last frame as a Hit with its rect: tabs, drag bars, borders, column bars, »/« buttons, title bars, close buttons, icons, icon grips, resize edges and dividers. It is always compiled, so an app can return it from a ui.docking control method next to the layout and the focused panel.

The testing feature adds docking::testing::DockDriver. It runs a DockArea in a headless egui context with real pointer and key events, and applies the actions each frame emits.

let mut driver = DockDriver::new(app_layout(), Panels::default());
let bar = driver.hit(&Hit::DragBar(PanelId::Layers)).unwrap();
driver.drag(bar.center(), egui::pos2(2.0, 300.0));
driver.settle();
assert!(driver.hit(&Hit::ColumnBar(PanelId::Layers)).is_some());

It also has press, release, click, hold, key, focus, hits, and checks for painted previews (filled, outlined).

craft-ui's own tests in crates/ui/tests/docking_workflows.rs:

• Load every app's v1 layout fixture, dock groups into a new dock, a bottom band and a seam, collapse and expand the column, float it by command and drag it back, then check the layout reads back as written.
• Render Layers, Properties and Brushes panels with real content, and merge, detach and redock Brushes by pointer at two widths, two scales and both themes. Set CRAFT_UI_DOCK_VISUAL_DIR to keep the captures:

CRAFT_UI_DOCK_VISUAL_DIR=/absolute/output/path cargo test -p craft-ui --test docking_workflows --all-features

An adopting app still tests its own commands, contents, saved workspaces and themes.

What stays in the app

• The panel registry, panel contents and document state.
• Named workspaces, Reset, and when and where layouts are saved.
• Menus and key bindings outside the docking chrome.
• Native window ownership on platforms without egui viewports.