Errors

The library draws one line. Runtime input is rejected with a value; programmer errors throw. Commands, saved state, drops and popup blockers are runtime input: they can be wrong at any moment in a running app, so they come back as events or result objects and never as exceptions. Constructing an invalid layout node, or registering a layout interpreter that returns garbage, is a bug in code, so it throws immediately with a message saying what was expected.

Rejected, not thrown

Where	Shape
update / wm.dispatch / wm.simulate	{ state: <unchanged>, events: [{ type: "command/rejected", command, id, reason }], effects: [] }
wm.load	returns null; subscribers get state/load-rejected { reason, version? }
migrate	{ ok: false, state: null, reason, version? }
validate (layout trees)	{ ok: false, errors: [{ path, message }] }
previewDrop, dropTargetAt	null
attachPopouts().popOut	a command/rejected result (popup-blocked, unknown-window, blocked), with state and DOM untouched
matchRules	an uncompilable titleRegex simply doesn’t match

Every rejection reason

Reason	Commands (or source)
handler-threw	update: an extension handler threw, or returned a value that is not { state, ... }
invalid-command	update: a non-object command, or one without a string type
unknown-command	update: no handler for type
invalid-id	update: a name field (id, target, to, parent, workspace, output, fallback) that is an Object.prototype key such as __proto__ or constructor
missing-id	window/create, workspace/create, workspace/rename (to), output/create
duplicate-id	window/create, workspace/create, workspace/rename, output/create
unknown-parent	window/create (any parent that is not an existing window, including NaN or "")
unknown-window	nearly every window/* command; scratchpad/toggle; focus subjects of swap-next/move-before/…; attachPopouts
unknown-workspace	window/create (after rules), window/move-to-workspace, workspace/activate, workspace/remove, workspace/move-to-output, every layout/* command except rotate-split
unknown-output	workspace/create, workspace/move-to-output, output/remove, output/focus
unknown-layer	window/create, window/set-layer
unknown-mode	window/create, window/set-mode
unknown-role	window/create
unknown-status	the status setters (internal guard; not reachable through the built-in commands)
unknown-zone	window/drop: not a DROP_ZONES value, or the interpreter maps it to no op
unknown-split	layout/resize-split
unknown-layout	the manager: layout/set, workspace/create, layout/toggle with a type no interpreter knows
invalid-layout	layout/set, layout/toggle
invalid-ratio	layout/set-ratio
invalid-geometry	window/move, window/resize
invalid-constraints	window/set-constraints
invalid-path	layout/resize-split
invalid-index	layout/resize-split, workspace/reorder, output/reorder
invalid-weights	layout/resize-split
missing-value	layout/resize-split
missing-weights	layout/resize-split (a delta on columns/rows before any sizes are stored)
not-resizable	layout/resize-split, layout/set-ratio
not-bsp	layout/rotate-split
invalid-config	config/set
invalid-rules	rules/set
invalid-urgent	window/set-urgent
invalid-sticky	window/set-sticky
no-urgent-window	focus/urgent
empty-scratchpad	scratchpad/toggle
not-scratchpad	scratchpad/toggle, window/from-scratchpad
not-on-workspace	window/promote, window/swap, window/focus, focus/urgent (a window hidden in the scratchpad, or a child of one)
popped-out	window/focus (a window that is popped out, or a child of one)
not-visible	window/focus (the target would still not be shown after the focus policy ran)
hidden-parent	window/create (the parent is hidden in the scratchpad)
parent-on-other-workspace	window/create (a modal dialog on another workspace than its parent)
has-parent	window/move-to-workspace, window/to-scratchpad (move the root ancestor instead)
popup-blocked	attachPopouts().popOut
last-workspace	workspace/remove
last-workspace-on-output	workspace/remove, workspace/move-to-output
last-output	output/remove
missing-workspaces	output/create
different-workspaces	window/swap, window/drop, window/move-before/after
same-window	window/drop, window/move-before/after
not-tiled	window/drop, window/detach, window/swap-next/previous, window/move-before/after
not-draggable	window/drop, window/detach
drag-disabled	window/drop (drag mode off), window/detach (toFloating: "off")
zone-disabled	window/drop
too-small	window/drop (tooSmall: "reject" with geometry)
blocked	window/drop, window/detach, window/pop-out
invalid-json, invalid-state, future-version, no-migration-path	wm.load (state/load-rejected) and migrate (except invalid-json)

What throws

Function	Throws	When
view	TypeError	the id is not a non-empty string
row, column, grid, stack, overlay, container	TypeError	options are not a plain object (including “a node where options were expected”: you forgot the options argument), a child is not a node, or the kind is unknown
place, size, gap, inset, anchor, modifier	TypeError	the same checks; anchor also requires a string options.to
fromJSON	TypeError, SyntaxError	an invalid tree (all errors listed), or invalid JSON text
transform (and every transform built on it)	TypeError	the input is not a node
derive / wm.present / a render	TypeError	no interpreter for the workspace’s layout type, or an interpreter that returned a non-node. The manager rejects unknown types up front to prevent this.
createState	TypeError	workspaces is empty
createResize	TypeError	an unknown edge
defineWindowAlgebraElement	Error	no customElements/HTMLElement available and none passed in
an extension handler	whatever it throws	update does not catch exceptions from your handlers; keep them pure and total

A built-in command handler that throws is a bug. The test suite asserts rejections for malformed input command by command, and explicitly asserts “never throws and never mutates state” (on deep-frozen state) for layout/resize-split and the docking-tree commands (test/update.test.mjs, test/tree.test.mjs).