fix(completion): keep the popup inside the window near screen edges

Anchoring the popup below the caret let it spill off the right/bottom of
the window when the caret sat at the edge, so the browser clipped it.
After render, measure the popup and shift it left off the right edge or
flip it above the caret line when there's no room below.

src/components/PlanEditor.vue

@@ -2,12 +2,14 @@
 import { ref, computed, onMounted, nextTick } from "vue"
 import type { HighlighterCore } from "shiki/core"
 import { getCompletions, type CompletionContext } from "./completion"
+import { fitPopup } from "./popupFit"
 
 const props = defineProps<{ modelValue: string; error: string | null }>()
 const emit = defineEmits<{ "update:modelValue": [value: string] }>()
 
 const textarea = ref<HTMLTextAreaElement>()
 const backdrop = ref<HTMLElement>()
+const popupEl = ref<HTMLElement>()
 const highlighter = ref<HighlighterCore>()
 
 // ── Completion popup state ───────────────────────────────────────────────────
@@ -75,10 +77,29 @@ function position() {
   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,
-  }
+  const caretLeft = metrics.padLeft + col * metrics.charWidth - el.scrollLeft
+  const caretTop = metrics.padTop + lineIndex * metrics.lineHeight - el.scrollTop
+  popup.value = { left: caretLeft, top: caretTop + metrics.lineHeight }
+  // The popup has no measurable size until Vue has rendered it; adjust once it does.
+  nextTick(() => keepInView(caretLeft, caretTop))
+}
+
+/** Nudge the popup back inside the window once it has a measurable size. */
+function keepInView(caretLeft: number, caretTop: number) {
+  const el = textarea.value
+  const list = popupEl.value
+  if (!el || !list) return
+  // popup left/top are relative to `.code`, which the textarea fills exactly, so
+  // the textarea rect bridges those local coordinates to the window.
+  const ta = el.getBoundingClientRect()
+  const box = list.getBoundingClientRect()
+  popup.value = fitPopup({
+    caret: { left: caretLeft, top: caretTop },
+    lineHeight: metrics.lineHeight,
+    box: { width: box.width, height: box.height },
+    origin: { left: ta.left, top: ta.top },
+    viewport: { width: window.innerWidth, height: window.innerHeight },
+  })
 }
 
 function accept(i: number) {
@@ -199,6 +220,7 @@ function syncScroll() {
       ></textarea>
       <ul
         v-if="completion"
+        ref="popupEl"
         class="completion"
         :style="{ left: popup.left + 'px', top: popup.top + 'px' }"
       >