komorebi/docs/dev/core-model.md

The core model: KDE and Windows compared, and the split into komorebi-core

Status: measurement and proposal (plan step 9 preparation, KDE side). Measured at the tip of wip-u-kfde (based on wip-main 633e7453); the Windows tree is komorebi/src, KDE is komorebi-for-kde/komorebi/src, macOS (komorebi-for-mac/komorebi/src) is listed only where it helps. Nothing here has been run on Windows or macOS.

1. Measured differences

"Code lines" drop // lines and blank lines and strip indentation, so KDE's much heavier doc comments do not count; the inline #[cfg(test)] modules are included. "Changed" is the number of </> lines of diff between the two stripped files (an insertion of n lines counts n, a replacement of n lines counts 2n). "Identical" is the number of stripped lines present in both in the same order (a common subsequence, so it is a lower bound on shared text).

file	code lines Windows / KDE / macOS	changed W vs K	changed K vs M	changed W vs M	identical W&K
container.rs	230 / 254 / 241	196	119	185	144
monitor.rs	643 / 666 / 675	613	205	616	348
workspace.rs	2203 / 2491 / 2196	2314	1097	2113	1190
window.rs	1134 / 916 / 1641	1168	1413	1767	441
window_manager.rs	4716 / 4023 / 4896	4543	1597	5434	2098

Whole-file raw diff sizes (comments included): container 271, monitor 752, workspace 3167, window 1520, window_manager 6358 changed lines (Windows vs KDE). ring.rs is already shared (crates/komorebi-collections, Ring, Lockable).

Two things stand out. First, every file KDE and macOS share is much closer to each other than either is to Windows (K vs M changed is a third to a half of W vs K for monitor, workspace, window_manager, static_config, state). Windows is the odd one out, mostly because it carries older and larger machinery (see section 2). Second, the structs themselves are almost the same: Workspace has 31 fields on both sides and they differ only in the order of work_area_offset_rules/work_area_offset; the differences are in the methods around them.

2. Platform-specific fields and methods, per layer

container (smallest)

• Same fields (id, locked, windows: Ring<Window>). Windows' windows is private and locked has #[serde(default)]; KDE's windows is pub.
• Window id type: hwnd: isize (Windows) vs id: u32 (KDE) in contains_window, idx_for_window, hide(omit).
• Windows only: hwnd_from_exe (KDE keeps only idx_from_exe), Window::restore_with_border / hide_with_border in load_focused_window (Windows' border and stackbar are separate windows). exe() returns Result on Windows, Option on KDE.
• After the converge revisions of this stack the signatures are the same (add_window(Window), hide(&self, Option<id>), restore(&self), load_focused_window(&mut self), no Result) and the item order is the same. What is left is the id type and the three Windows-only lines above.

monitor

• Windows only fields: name, device_id, workspace_names: HashMap<usize, String>; serial_number_id is Option<String> (KDE: String); id is isize (KDE u32).
• Windows only methods: none in the public API; KDE has assign_windows_to_workspaces (it pushes the workspace of each window to the compositor's desktops, Backend::set_window_workspace).
• Behaviour that differs: Windows load_focused_workspace restores/hides workspaces itself, KDE asks the compositor to switch desktops (Backend::switch_workspace) and tiles what is in front; the Windows update_workspaces_globals pushes padding/border/work area into every workspace (WorkspaceGlobals), KDE calls update_workspace_globals(idx, offset) for the focused one. KDE has a MonitorInformation with 4 fields, Windows 6.
• BORDER_ENABLED (Windows) vs BORDER_DRAWN (KDE): the compositor draws borders.

workspace

• Windows only methods: windows_on_show, border_kinds_on_show, hwnd_from_exe.
• KDE only methods: adopt_maximized_window, focus_floating_window, focus_previous_floating_window, prune_duplicate_windows, reap_invalid_windows, remove_floating_window (my reading, not verified: KDE learns of closes and moves through the bridge snapshot, so the workspace reaps and prunes).
• 42 of 45/48 pub fn names are common. Windows restore/hide take the monitor handle and wallpaper and do the showing and hiding (WindowsApi, 8 mentions in the file); KDE leaves showing to the compositor (5 backend() calls).
• Scrolling layout glue (scrolling.rs): 1328 lines KDE, 1315 macOS, ~2200 Windows; measured first, not part of this comparison.

window (not shared: the platform handle)

• Window { hwnd: isize } (Windows, methods that call WindowsApi, 1134 code lines) vs Window { id: u32 } (KDE, Copy, every method calls backend(), 916 code lines) vs macOS (AX element, application, observer; not Copy, 1641 code lines).
• Both Windows and KDE Window are Copy handles, which is what lets the container and workspace code hold windows by value. macOS is not (clone).
• Window is where the split goes: the core must not call a method of Window that does not exist on all three.

window_manager

• Windows only fields: known_hwnds, virtual_desktop_id, uncloack_to_ignore, has_pending_raise_op, pending_move_op with a handle and rect, already_moved_window_handles of isize.
• KDE only fields: opacity_applied, minimized_windows, pending_resize_op, live_resize, known_window_ids (a u32 map); pending_move_op has no rect.
• 43 WindowsApi mentions in Windows' window_manager.rs, 15 backend() in KDE's. Every one of them becomes a Backend call or a Window method in the core.
• process_command and process_event are not in this table (see section 4 and the plan).

3. Proposed split: komorebi-core

komorebi-core                 (new crate, no platform dependency)
  ring, container, monitor, workspace, window_manager      (generic)
  trait Platform: Backend + { type Window; hooks }         (what is not in Backend)
komorebi-backend              (exists: Backend, WindowId, MonitorId, WindowInfo, Error, Capability)
komorebi (windows) / (kde) / (mac)
  Window (the platform handle), the Backend implementation, event sources, borders, ...

Generic over what:

• WindowManager<P: Platform>, with Platform::Window: Copy + Eq + Hash + Serialize + Deserialize and fn id(&self) -> WindowId. The three id types (isize, u32, u32) disappear into WindowId (WindowId::from_hwnd/from_u32, already in komorebi-backend). macOS needs Clone instead of Copy: the generic code has to take &P::Window and clone, or macOS keeps a Copy handle and an id registry as the seam doc proposes (docs/dev/seam.md section 7, consequence 1).
• Backend is a trait object or a type parameter; docs/dev/seam.md and the KDE conversion show it can be the shared trait unchanged, plus KDE's extension (drag_*, look, wallpaper) behind Platform hooks.
• Methods that are still platform behaviour (Window::hide_with_border, showing and hiding a workspace on Windows vs asking for a desktop on KDE) move behind two hooks: Platform::show_workspace(monitor, idx) and Platform::hide_workspace(...), with the Windows implementation hiding/restoring windows and the KDE one calling Backend::switch_workspace.

Exact order of layers (bottom-up, as in the plan)

1. ring is done (komorebi-collections).
2. container (230 lines): needs Window generic and id(). Smallest; do first. Windows must rename hwnd parameters to WindowId or an id() accessor and give exe() the KDE return type (Option<String>) or both use Result.
3. monitor (643): needs Workspace generic, MonitorId, and the Platform show/hide hooks. Windows' extra fields (name, device_id, workspace_names) stay in the shared struct as Option/default fields or become a P::MonitorExtra associated type; KDE serial_number_id becomes Option<String> with "" mapped to None.
4. workspace (2203): by far the biggest; the platform split is restore/hide/windows_on_show/ border_kinds_on_show (Windows) and reap/prune/adopt_maximized (KDE). Scrolling glue is measured before it moves.
5. window_manager (4716): after the first three are shared. The platform fields (section 2) go in a P::State associated type.
6. the flat router and process_event (separate steps in the plan).

4. What KDE has done (this stack) and what Windows must do before the crate exists

Landed on wip-u-kfde (all native tests green, none of it run on a VM):

• KDE's Backend extends the shared trait; ids, errors and info types are the shared ones at the trait; the core still holds Window { id: u32 } and converts at the call.
• container.rs and monitor.rs are in the same order as Windows', and Container has the Windows signatures (add_window(Window), infallible hide, restore, load_focused_window).
• All process_command arms longer than 20 lines are named methods (list in section 5).

I did not create the shared crate: the Windows tree still has isize handles in Container, Monitor::id: isize, and calls WindowsApi directly in 43 places of window_manager.rs. Windows (kfw) has to:

1. Finish adopting Backend in window_manager.rs, workspace.rs, monitor.rs (the 8 + 43 + 2 WindowsApi call sites there), so no core file names WindowsApi.
2. Give Container the same API KDE now has: accept a WindowId (or Window::id()), return Option from exe(), and drop hwnd_from_exe or move it to a platform extension.
3. Make Monitor.id and serial_number_id the shared types (MonitorId, Option<String>), which changes what komorebic state prints for Windows (a BEHAVIOUR change for the JSON; a test must pin it).
4. Convert its own multi-line process_command arms (the plan counts 22-23 per platform) into named methods with the same names KDE used (section 5), so the router can later be shared.

5. process_command: arms over 20 lines (KDE, before conversion)

arm	lines	now
ToggleWorkspaceLayer	90	toggle_workspace_layer()
EagerFocus	75	eager_focus(exe)
ResizeWindowAxis	60	resize_window_axis(axis, sizing)
IgnoreRule	55	add_ignore_rule(identifier, id)
Query	51	answer_query(query, reply)
SessionFloatRule	39	add_session_float_rule()
ReplaceConfiguration	34	replace_configuration(config)
CycleFocusEmptyWorkspace	32	cycle_focus_empty_workspace(direction)
CloseWorkspace	29	close_workspace()
MoveContainerToLastWorkspace	28	move_container_to_last_workspace(follow)
SendContainerToLastWorkspace	27	same method, follow = false
CycleFocusWorkspace	26	cycle_focus_workspace(direction)
LayoutRatios	23	set_layout_ratios(columns, rows)
BorderColour	21	set_border_colour(kind, r, g, b)
NamedWorkspaceRule	21	add_named_workspace_rule(identifier, id, workspace)
InitialNamedWorkspaceRule	21	add_initial_named_workspace_rule(identifier, id, workspace)
ToggleWorkspaceWindowContainerBehaviour	21	toggle_workspace_window_container_behaviour()

All are pub fn name(&mut self, ...) -> eyre::Result<()> in impl WindowManager (process_command.rs), except set_border_colour, which cannot fail.