feat(words): "+ between blocks" hover-zone inserter (POLISH-2b)

Fixes the wedge case the user just hit: two atomic blocks back-to-back
(image, table, code) leave no caret position between them, so there's
no way to insert a paragraph by clicking. Now: hover near the seam
between any two top-level blocks and a "+" appears on the left margin
with a faint accent line spanning the seam. Click "+" → a new
paragraph is dropped at that index and the caret lands at the start
so the user starts typing immediately.

Same overlay pattern as the block-handle: single component watches
mousemove inside `[data-words-content]`, computes the boundaries of
all top-level blocks (plus "before first" and "after last" seams),
snaps to the closest seam within 18px of the cursor's Y, and renders
a fixed-positioned strip with the "+" button and accent line.

New engine surface:
- `insertBlockAt(state, blockIndex, block)` in `operations.ts` —
  splices the block in, normalizes, drops caret at `[blockIndex, 0]`.
  `blockIndex === children.length` appends to the tail.
- `insertBlock` case added to `WordsCommand` + dispatcher in
  `commands.ts`. The command surface keeps `block` as a loose JSON
  record so consumers don't need to import `WordsBlock` — the
  reducer casts on the boundary and `normalizeDocument` validates.

Verified in browser: hover at the seam between code (index 3) and
image (index 4) → "+" appears at the left margin → click → doc
becomes 0:heading | 1:paragraph | 2:quote | 3:code | 4:paragraph |
5:image; caret in the new paragraph ready for input.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/uix/eidos/components/words/index.ts

@@ -16,6 +16,7 @@ import FindReplace from './words-find-replace.svelte';
 import Drawer from './words-drawer.svelte';
 import ImageFloatBar from './words-image-float-bar.svelte';
 import BlockHandle from './words-block-handle.svelte';
+import BlockInserter from './words-block-inserter.svelte';
 
 type WordsNamespace = typeof WordsComponent & {
 	Content: typeof Content;
@@ -35,6 +36,7 @@ type WordsNamespace = typeof WordsComponent & {
 	Drawer: typeof Drawer;
 	ImageFloatBar: typeof ImageFloatBar;
 	BlockHandle: typeof BlockHandle;
+	BlockInserter: typeof BlockInserter;
 };
 
 const Words = WordsComponent as WordsNamespace;
@@ -55,6 +57,7 @@ Words.FindReplace = FindReplace;
 Words.Drawer = Drawer;
 Words.ImageFloatBar = ImageFloatBar;
 Words.BlockHandle = BlockHandle;
+Words.BlockInserter = BlockInserter;
 
 export { Words };
 export default Words;

src/uix/eidos/components/words/words-block-inserter.svelte

@@ -0,0 +1,150 @@
+<script lang="ts">
+	/**
+	 * Words.BlockInserter — "+" hover-zone between blocks (Notion-style).
+	 *
+	 * Solves the wedge problem: with atomic blocks (image, table, code)
+	 * back-to-back there's no caret position between them, so the user
+	 * can't insert a paragraph by clicking. This overlay watches mouse
+	 * Y inside `[data-words-content]`, finds the nearest seam between
+	 * two adjacent top-level blocks (plus the seams before the first
+	 * and after the last), and renders a horizontal hover-line with a
+	 * floating "+" on the left margin.
+	 *
+	 * Click → `snippet.applyCommand({ type: 'insertBlock', blockIndex,
+	 * block: createParagraph() })`. Caret lands at the start of the
+	 * new paragraph so the user types straight away.
+	 *
+	 * Single overlay reused across all seams — same pattern as the
+	 * block-handle and image-float-bar. No sema (these are discrete
+	 * imperatives, not perceptual events).
+	 */
+	import { Plus } from '$uix/eidos/components/icon';
+	import type { ProviderSnippetProps } from '$soma/components/words';
+
+	let { snippet }: { snippet: ProviderSnippetProps } = $props();
+
+	type Seam = {
+		readonly insertIndex: number;
+		readonly y: number;
+		readonly left: number;
+		readonly right: number;
+	};
+
+	// Seam under the cursor (snapped to the closest one within
+	// `SEAM_RANGE_PX`). null while the cursor is too far from any seam.
+	let seam = $state<Seam | null>(null);
+
+	const SEAM_RANGE_PX = 18;
+
+	function listBlockBoundaries(content: HTMLElement): Seam[] {
+		// Top-level blocks only — same predicate as the block-handle.
+		const blocks = Array.from(
+			content.querySelectorAll<HTMLElement>(
+				'[data-words-node="block"][data-words-path]'
+			)
+		).filter((el) => !(el.getAttribute('data-words-path') ?? '').includes('.'));
+		if (!blocks.length) return [];
+		const seams: Seam[] = [];
+		// "Before first block" seam.
+		const first = blocks[0].getBoundingClientRect();
+		seams.push({ insertIndex: 0, y: first.top, left: first.left, right: first.right });
+		// Between-blocks seams: anchor at midpoint of the visual gap.
+		for (let i = 0; i < blocks.length - 1; i++) {
+			const a = blocks[i].getBoundingClientRect();
+			const b = blocks[i + 1].getBoundingClientRect();
+			seams.push({
+				insertIndex: i + 1,
+				y: (a.bottom + b.top) / 2,
+				left: Math.min(a.left, b.left),
+				right: Math.max(a.right, b.right)
+			});
+		}
+		// "After last block" seam.
+		const last = blocks[blocks.length - 1].getBoundingClientRect();
+		seams.push({
+			insertIndex: blocks.length,
+			y: last.bottom,
+			left: last.left,
+			right: last.right
+		});
+		return seams;
+	}
+
+	function findSeamForCursor(e: MouseEvent, content: HTMLElement): Seam | null {
+		const seams = listBlockBoundaries(content);
+		let best: Seam | null = null;
+		let bestDelta = SEAM_RANGE_PX;
+		for (const s of seams) {
+			const delta = Math.abs(s.y - e.clientY);
+			if (delta < bestDelta && e.clientX >= s.left - 32 && e.clientX <= s.right + 32) {
+				best = s;
+				bestDelta = delta;
+			}
+		}
+		return best;
+	}
+
+	$effect(() => {
+		const content = document.querySelector<HTMLElement>('[data-words-content]');
+		if (!content) return;
+
+		function onmove(e: MouseEvent) {
+			seam = findSeamForCursor(e, content);
+		}
+		function onleave() {
+			seam = null;
+		}
+		content.addEventListener('mousemove', onmove);
+		content.addEventListener('mouseleave', onleave);
+
+		const win = content.ownerDocument.defaultView;
+		const onscroll = () => {
+			// Cursor stays put on scroll; recompute under the LAST known
+			// pointer would require tracking the cursor. Simplest: hide
+			// on scroll and let the next mousemove re-snap.
+			seam = null;
+		};
+		win?.addEventListener('scroll', onscroll, { passive: true });
+		win?.addEventListener('resize', onscroll, { passive: true });
+
+		return () => {
+			content.removeEventListener('mousemove', onmove);
+			content.removeEventListener('mouseleave', onleave);
+			win?.removeEventListener('scroll', onscroll);
+			win?.removeEventListener('resize', onscroll);
+		};
+	});
+
+	function insertHere() {
+		if (!seam) return;
+		const blockIndex = seam.insertIndex;
+		snippet.applyCommand({
+			type: 'insertBlock',
+			blockIndex,
+			// Plain JSON paragraph — normalizeDocument fills in defaults
+			// (`children`, marks) and validates the shape.
+			block: { type: 'paragraph', children: [{ type: 'text', text: '' }] }
+		});
+		seam = null;
+	}
+</script>
+
+{#if seam}
+	<div
+		data-words-block-inserter
+		style="top: {seam.y}px; left: {seam.left}px; width: {seam.right - seam.left}px;"
+		role="presentation"
+	>
+		<button
+			type="button"
+			data-words-block-inserter-button
+			title="Insert paragraph here"
+			aria-label="Insert paragraph here"
+			onclick={insertHere}
+			onmousedown={(e) => e.preventDefault()}
+		>
+			<Plus size="xs" decorative />
+		</button>
+		<span data-words-block-inserter-line aria-hidden="true"></span>
+	</div>
+{/if}

src/uix/eidos/components/words/words.css

@@ -733,6 +733,63 @@
 	background: var(--words-toolbar-border);
 }
 
+/* Block inserter — hover-zone between blocks. Solves the wedge case
+   where two atomic blocks (image, table, code) are back-to-back and
+   no caret position exists between them. The wrapper is a thin
+   horizontal strip anchored at the seam between two blocks; it
+   carries a faint line and a left-margin "+" button. Click → insert
+   a paragraph at that position. */
+[data-words-block-inserter] {
+	position: fixed;
+	pointer-events: none;
+	transform: translateY(-50%);
+	display: flex;
+	align-items: center;
+	z-index: var(--z-index-popover);
+}
+
+[data-words-block-inserter-button] {
+	pointer-events: auto;
+	position: absolute;
+	left: -28px;
+	display: inline-flex;
+	align-items: center;
+	justify-content: center;
+	inline-size: 1.5rem;
+	block-size: 1.5rem;
+	padding: 0;
+	border: var(--words-border-width) solid var(--words-toolbar-border);
+	border-radius: 999px;
+	background: var(--words-toolbar-bg);
+	color: var(--words-status-color);
+	cursor: pointer;
+	box-shadow: var(--words-shadow);
+	transition:
+		background 120ms ease,
+		color 120ms ease,
+		border-color 120ms ease;
+}
+
+[data-words-block-inserter-button]:hover {
+	background: var(--_words-accent-solid);
+	border-color: var(--_words-accent-solid);
+	color: var(--_words-accent-on);
+}
+
+[data-words-block-inserter-line] {
+	pointer-events: auto;
+	flex: 1 1 auto;
+	block-size: 2px;
+	border-radius: 1px;
+	background: color-mix(in srgb, var(--_words-accent-solid) 35%, transparent);
+	opacity: 0;
+	transition: opacity 120ms ease;
+}
+
+[data-words-block-inserter]:hover [data-words-block-inserter-line] {
+	opacity: 1;
+}
+
 [data-words-drawer-section-title] {
 	display: flex;
 	align-items: baseline;

src/uix/soma/components/words/engine/commands.ts

@@ -30,6 +30,7 @@ import {
 	insertParagraph,
 	deleteBlockAt,
 	duplicateBlockAt,
+	insertBlockAt,
 	insertImage,
 	insertTable,
 	insertTableColumn,
@@ -125,6 +126,17 @@ export type WordsCommand =
 	| { type: 'moveBlock'; blockIndex: number; direction: 'up' | 'down' }
 	/** Insert a deep copy of the block immediately after it. */
 	| { type: 'duplicateBlock'; blockIndex: number }
+	/**
+	 * Insert an arbitrary block at a position. Caret lands at the start
+	 * of the new block. Used by the block-inserter overlay to drop a
+	 * paragraph between atomic blocks (image / table / code) where no
+	 * caret position exists.
+	 */
+	| {
+			type: 'insertBlock';
+			blockIndex: number;
+			block: Readonly<Record<string, unknown>>;
+	  }
 	| { type: 'insertTableRow'; position?: 'before' | 'after' }
 	| { type: 'insertTableColumn'; position?: 'before' | 'after' }
 	| { type: 'deleteTableRow' }
@@ -185,6 +197,16 @@ export function applyWordsCommand(
 			return moveBlockAt(state, command.blockIndex, command.direction);
 		case 'duplicateBlock':
 			return duplicateBlockAt(state, command.blockIndex);
+		case 'insertBlock':
+			// Reducer accepts a typed WordsBlock; the command surface keeps
+			// it loose so consumers (overlays / extensions) can pass JSON
+			// without importing the full block union. Cast on the boundary
+			// — normalizeDocument enforces shape downstream.
+			return insertBlockAt(
+				state,
+				command.blockIndex,
+				command.block as unknown as Parameters<typeof insertBlockAt>[2]
+			);
 		case 'insertTableRow':
 			return insertTableRow(state, { position: command.position });
 		case 'insertTableColumn':

src/uix/soma/components/words/engine/operations.ts

@@ -907,6 +907,46 @@ export function deleteBlockAt(
 	};
 }
 
+/**
+ * Insert an arbitrary block at `blockIndex`. The new block takes that
+ * index; everything from `blockIndex` onwards shifts down one slot.
+ * `blockIndex === children.length` appends to the end. Caret lands at
+ * the start of the newly-inserted block, so the user can start typing
+ * straight away.
+ *
+ * Used by the block-inserter overlay to drop a paragraph between two
+ * atomic blocks (image, table, code) where no caret position exists.
+ */
+export function insertBlockAt(
+	state: WordsEditorState,
+	blockIndex: number,
+	block: WordsBlock
+): WordsOperationResult {
+	const children = state.document.children;
+	if (blockIndex < 0 || blockIndex > children.length) {
+		return { state, changed: false };
+	}
+	const nextChildren = [
+		...children.slice(0, blockIndex),
+		block,
+		...children.slice(blockIndex)
+	];
+	const normalized = normalizeDocument({
+		...state.document,
+		children: nextChildren
+	}).document;
+	const point = pointFromInlineTextOffset(normalized, [blockIndex], 0);
+	const selection = createCollapsedSelection(point.path, point.offset);
+	return {
+		state: {
+			document: normalized,
+			selection,
+			activeMarks: getActiveMarksForSelection(normalized, selection)
+		},
+		changed: true
+	};
+}
+
 export function insertImage(
 	state: WordsEditorState,
 	options: {

web/routes/uix/components/words/+page.svelte

@@ -1079,6 +1079,7 @@
 					{/if}
 					<Words.ImageFloatBar snippet={snippetProps} />
 					<Words.BlockHandle snippet={snippetProps} />
+					<Words.BlockInserter snippet={snippetProps} />
 					{#if showStatus}
 						<Words.Status>
 							{#if invalid}