feat(editor): show a completion popup as you type

Anchor a suggestion list at the caret (monospace metrics, re-anchored on
scroll), with arrow/Enter/Tab/Esc handling and click-to-accept. Accepting
a key like `status` chains straight into its value suggestions.

src/components/PlanEditor.vue

@@ -1,6 +1,7 @@
 <script setup lang="ts">
-import { ref, computed, onMounted } from 'vue'
+import { ref, computed, onMounted, nextTick } from 'vue'
 import type { HighlighterCore } from 'shiki/core'
+import { getCompletions, type CompletionContext } from './completion'
 
 const props = defineProps<{ modelValue: string; error: string | null }>()
 const emit = defineEmits<{ 'update:modelValue': [value: string] }>()
@@ -9,7 +10,18 @@ const textarea = ref<HTMLTextAreaElement>()
 const backdrop = ref<HTMLElement>()
 const highlighter = ref<HighlighterCore>()
 
+// ── Completion popup state ───────────────────────────────────────────────────
+const completion = ref<CompletionContext | null>(null)
+const selected = ref(0)
+const popup = ref({ left: 0, top: 0 })
+// Suppress the popup for one keystroke after Escape, so it doesn't pop straight
+// back open while the author is still typing the word they just dismissed.
+let justEscaped = false
+// Pixel metrics of the (monospace, non-wrapping) textarea, measured once.
+let metrics = { charWidth: 8, lineHeight: 20, padLeft: 16, padTop: 14 }
+
 onMounted(async () => {
+  measure()
   // Lazy-load Shiki as its own chunk so it stays out of the initial bundle.
   // Fine-grained: only the TOML grammar + one light theme, JS engine (no WASM).
   const [core, engine, toml, theme] = await Promise.all([
@@ -25,6 +37,93 @@ onMounted(async () => {
   })
 })
 
+function measure() {
+  const el = textarea.value
+  if (!el) return
+  const cs = getComputedStyle(el)
+  const ctx = document.createElement('canvas').getContext('2d')!
+  ctx.font = `${cs.fontSize} ${cs.fontFamily}`
+  const lh = parseFloat(cs.lineHeight) // px value in every modern browser
+  metrics = {
+    charWidth: ctx.measureText('0000000000').width / 10,
+    lineHeight: Number.isNaN(lh) || lh < 4 ? parseFloat(cs.fontSize) * 1.6 : lh,
+    padLeft: parseFloat(cs.paddingLeft),
+    padTop: parseFloat(cs.paddingTop),
+  }
+}
+
+/** Recompute suggestions from the live textarea value + caret. */
+function refresh() {
+  const el = textarea.value
+  if (!el) return
+  completion.value = justEscaped ? null : getCompletions(el.value, el.selectionStart)
+  selected.value = 0
+  if (completion.value) position()
+}
+
+/** Place the popup just below the caret (monospace ⇒ column × charWidth). */
+function position() {
+  const el = textarea.value
+  if (!el) return
+  const before = el.value.slice(0, el.selectionStart)
+  const lineIndex = before.split('\n').length - 1
+  const line = before.slice(before.lastIndexOf('\n') + 1)
+  let col = 0
+  for (const ch of line) col = ch === '\t' ? col + (2 - (col % 2)) : col + 1
+  popup.value = {
+    left: metrics.padLeft + col * metrics.charWidth - el.scrollLeft,
+    top: metrics.padTop + (lineIndex + 1) * metrics.lineHeight - el.scrollTop,
+  }
+}
+
+function accept(i: number) {
+  const el = textarea.value
+  const ctx = completion.value
+  if (!el || !ctx) return
+  const item = ctx.items[i]
+  const caret = ctx.from + item.insert.length
+  completion.value = null
+  emit('update:modelValue', el.value.slice(0, ctx.from) + item.insert + el.value.slice(ctx.to))
+  nextTick(() => {
+    el.setSelectionRange(caret, caret)
+    el.focus()
+    refresh() // chain, e.g. `status = ` immediately offers the enum values
+  })
+}
+
+function onKeydown(e: KeyboardEvent) {
+  if (e.key !== 'Escape') justEscaped = false
+  const ctx = completion.value
+  if (!ctx) return
+  switch (e.key) {
+    case 'ArrowDown':
+      e.preventDefault()
+      selected.value = (selected.value + 1) % ctx.items.length
+      break
+    case 'ArrowUp':
+      e.preventDefault()
+      selected.value = (selected.value - 1 + ctx.items.length) % ctx.items.length
+      break
+    case 'Enter':
+    case 'Tab':
+      e.preventDefault()
+      accept(selected.value)
+      break
+    case 'Escape':
+      e.preventDefault()
+      completion.value = null
+      justEscaped = true
+      break
+    // Caret jumps would leave the popup stranded at a stale spot — close it.
+    case 'ArrowLeft':
+    case 'ArrowRight':
+    case 'Home':
+    case 'End':
+      completion.value = null
+      break
+  }
+}
+
 const highlighted = computed(() => {
   // a trailing newline needs a trailing char so the last backdrop line keeps height
   const code = props.modelValue.endsWith('\n') ? props.modelValue + ' ' : props.modelValue
@@ -39,12 +138,14 @@ function escapeHtml(s: string): string {
 
 function onInput(e: Event) {
   emit('update:modelValue', (e.target as HTMLTextAreaElement).value)
+  refresh()
 }
 
 function syncScroll() {
   if (!textarea.value || !backdrop.value) return
   backdrop.value.scrollTop = textarea.value.scrollTop
   backdrop.value.scrollLeft = textarea.value.scrollLeft
+  if (completion.value) position()
 }
 </script>
 
@@ -63,7 +164,24 @@ function syncScroll() {
         aria-label="Macroplan TOML source"
         @input="onInput"
         @scroll="syncScroll"
+        @keydown="onKeydown"
+        @blur="completion = null"
       ></textarea>
+      <ul
+        v-if="completion"
+        class="completion"
+        :style="{ left: popup.left + 'px', top: popup.top + 'px' }"
+      >
+        <li
+          v-for="(item, i) in completion.items"
+          :key="item.label"
+          :class="{ active: i === selected }"
+          @mousedown.prevent="accept(i)"
+        >
+          <span class="label">{{ item.label }}</span>
+          <span v-if="item.detail" class="detail">{{ item.detail }}</span>
+        </li>
+      </ul>
     </div>
     <div v-if="error" class="editor-error" role="alert">
       <span class="font-semibold">Can’t parse:</span> {{ error }}
@@ -133,6 +251,42 @@ function syncScroll() {
 .input::selection {
   background: color-mix(in oklch, var(--color-primary) 28%, transparent);
 }
+/* Completion popup, anchored just below the caret (see position()). */
+.completion {
+  position: absolute;
+  z-index: 2;
+  margin: 0;
+  padding: 0.25rem;
+  list-style: none;
+  min-width: 13rem;
+  max-width: 22rem;
+  max-height: 14rem;
+  overflow-y: auto;
+  background: var(--color-base-100);
+  border: 1px solid var(--color-base-300);
+  border-radius: 0.4rem;
+  box-shadow: 0 8px 24px color-mix(in oklch, black 16%, transparent);
+}
+.completion li {
+  display: flex;
+  align-items: baseline;
+  justify-content: space-between;
+  gap: 1.25rem;
+  padding: 0.2rem 0.5rem;
+  border-radius: 0.25rem;
+  cursor: pointer;
+  white-space: nowrap;
+}
+.completion li.active {
+  background: color-mix(in oklch, var(--color-primary) 18%, transparent);
+}
+.completion .label {
+  font-size: 0.78rem;
+}
+.completion .detail {
+  font-size: 0.68rem;
+  color: color-mix(in oklch, var(--color-base-content) 55%, transparent);
+}
 .editor-error {
   padding: 0.6rem 1rem;
   font-size: 0.74rem;