feat(vom): vom base algorithm and infrastructure

packages/vom/src/layers.ts

@@ -0,0 +1,79 @@
+import type { BlockingLayer, Rect, Viewport, VomNode } from "./types";
+
+/** Default fraction of the viewport an overlay must cover to block. */
+export const BLOCK_COVERAGE_THRESHOLD = 0.6;
+/** At/above this coverage with no inputs we call it an opaque mask. */
+export const MASK_COVERAGE_THRESHOLD = 0.9;
+
+const POSITIONED = new Set(["fixed", "absolute", "sticky"]);
+const FORM_TAGS = new Set(["input", "textarea", "select"]);
+
+export function coverage(rect: Rect | null, vp: Viewport): number {
+  if (!rect || vp.width <= 0 || vp.height <= 0) return 0;
+  const ix = Math.max(0, Math.min(rect.x + rect.w, vp.width) - Math.max(rect.x, 0));
+  const iy = Math.max(0, Math.min(rect.y + rect.h, vp.height) - Math.max(rect.y, 0));
+  const overlap = ix * iy;
+  if (overlap <= 0) return 0;
+  return Math.min(1, overlap / (vp.width * vp.height));
+}
+
+function isBlockingCandidate(node: VomNode): boolean {
+  if (node.pointerEvents === "none") return false;
+  return POSITIONED.has(node.position);
+}
+
+function hasModalFeature(node: VomNode): boolean {
+  return (
+    node.modal === true ||
+    node.tag === "dialog" ||
+    node.role === "dialog" ||
+    node.role === "alertdialog"
+  );
+}
+
+function classifyLayer(
+  nodes: VomNode[],
+  members: Set<number>,
+  blockerCoverage: number,
+): BlockingLayer["kind"] {
+  for (const node of nodes) {
+    if (!members.has(node.id)) continue;
+    if (hasModalFeature(node)) return "modal";
+    if (FORM_TAGS.has(node.tag)) return "modal";
+    if (node.tag === "iframe") return "modal";
+  }
+  return blockerCoverage >= MASK_COVERAGE_THRESHOLD ? "mask" : "modal";
+}
+
+export function detectBlockingLayer(nodes: VomNode[], vp: Viewport): BlockingLayer | null {
+  let blocker: { node: VomNode; coverage: number } | null = null;
+
+  for (const node of nodes) {
+    if (!isBlockingCandidate(node)) continue;
+    const cov = coverage(node.rect, vp);
+    const qualifies = cov >= BLOCK_COVERAGE_THRESHOLD || (hasModalFeature(node) && cov >= 0.15);
+    if (!qualifies) continue;
+    if (
+      blocker === null ||
+      cov > blocker.coverage ||
+      (cov === blocker.coverage && node.paintOrder > blocker.node.paintOrder)
+    ) {
+      blocker = { node, coverage: cov };
+    }
+  }
+
+  if (!blocker) return null;
+
+  const threshold = blocker.node.paintOrder;
+  const members = new Set<number>();
+  for (const node of nodes) {
+    if (node.paintOrder >= threshold) members.add(node.id);
+  }
+
+  return {
+    rootId: blocker.node.id,
+    kind: classifyLayer(nodes, members, blocker.coverage),
+    coverage: blocker.coverage,
+    members,
+  };
+}