feat(app): better diff/code comments (#14621)

Co-authored-by: adamelmore <2363879+adamdottv@users.noreply.github.com>
Co-authored-by: David Hill <iamdavidhill@gmail.com>

packages/ui/src/pierre/comment-hover.ts

@@ -0,0 +1,74 @@
+export type HoverCommentLine = {
+  lineNumber: number
+  side?: "additions" | "deletions"
+}
+
+export function createHoverCommentUtility(props: {
+  label: string
+  getHoveredLine: () => HoverCommentLine | undefined
+  onSelect: (line: HoverCommentLine) => void
+}) {
+  if (typeof document === "undefined") return
+
+  const button = document.createElement("button")
+  button.type = "button"
+  button.ariaLabel = props.label
+  button.textContent = "+"
+  button.style.width = "20px"
+  button.style.height = "20px"
+  button.style.display = "flex"
+  button.style.alignItems = "center"
+  button.style.justifyContent = "center"
+  button.style.border = "none"
+  button.style.borderRadius = "var(--radius-md)"
+  button.style.background = "var(--icon-interactive-base)"
+  button.style.color = "var(--white)"
+  button.style.boxShadow = "var(--shadow-xs)"
+  button.style.fontSize = "14px"
+  button.style.lineHeight = "1"
+  button.style.cursor = "pointer"
+  button.style.position = "relative"
+  button.style.left = "30px"
+  button.style.top = "calc((var(--diffs-line-height, 24px) - 20px) / 2)"
+
+  let line: HoverCommentLine | undefined
+
+  const sync = () => {
+    const next = props.getHoveredLine()
+    if (!next) return
+    line = next
+  }
+
+  const loop = () => {
+    if (!button.isConnected) return
+    sync()
+    requestAnimationFrame(loop)
+  }
+
+  const open = () => {
+    const next = props.getHoveredLine() ?? line
+    if (!next) return
+    props.onSelect(next)
+  }
+
+  requestAnimationFrame(loop)
+  button.addEventListener("mouseenter", sync)
+  button.addEventListener("mousemove", sync)
+  button.addEventListener("pointerdown", (event) => {
+    event.preventDefault()
+    event.stopPropagation()
+    sync()
+  })
+  button.addEventListener("mousedown", (event) => {
+    event.preventDefault()
+    event.stopPropagation()
+    sync()
+  })
+  button.addEventListener("click", (event) => {
+    event.preventDefault()
+    event.stopPropagation()
+    open()
+  })
+
+  return button
+}