fix: broken, infinitely looping minPresenceAhead calculation

.changeset/mighty-birds-eat.md

@@ -0,0 +1,5 @@
+---
+'@react-pdf/layout': minor
+---
+
+Rework minPresenceAhead detection and add tests

packages/layout/src/node/shouldBreak.js

@@ -1,46 +1,47 @@
 /* eslint-disable no-continue */
 
 import getWrap from './getWrap';
-import getNodesHeight from './getNodesHeight';
 
 const getBreak = node => node.props?.break || false;
 
-const getMinPresenceAhead = node => node.props?.minPresenceAhead;
+const getMinPresenceAhead = node => node.props?.minPresenceAhead || 0;
 
-const defaultPresenceAhead = element => height =>
-  Math.min(element.box.height, height);
+const getFurthestEnd = elements =>
+  Math.max(...elements.map(node => node.box.top + node.box.height));
 
-const getPresenceAhead = (elements, height) => {
-  let result = 0;
-
-  for (let i = 0; i < elements.length; i += 1) {
-    const element = elements[i];
-
-    if (!element.box) continue;
-
-    const isElementInside = height > element.box.top;
-    const presenceAhead =
-      element.props.presenceAhead || defaultPresenceAhead(element);
-
-    if (element && isElementInside) {
-      result += presenceAhead(height - element.box.top);
-    }
-  }
+const getEndOfMinPresenceAhead = child => {
+  return (
+    child.box.top +
+    child.box.height +
+    child.box.marginBottom +
+    getMinPresenceAhead(child)
+  );
+};
 
-  return result;
+const getEndOfPresence = (child, futureElements) => {
+  const afterMinPresenceAhead = getEndOfMinPresenceAhead(child);
+  const endOfFurthestFutureElement = getFurthestEnd(
+    futureElements.filter(node => !node.props?.fixed),
+  );
+  return Math.min(afterMinPresenceAhead, endOfFurthestFutureElement);
 };
 
 const shouldBreak = (child, futureElements, height) => {
-  const minPresenceAhead = getMinPresenceAhead(child);
-  const presenceAhead = getPresenceAhead(futureElements, height);
-  const futureHeight = getNodesHeight(futureElements);
+  if (child.props?.fixed) return false;
+
   const shouldSplit = height < child.box.top + child.box.height;
-  const shouldWrap = getWrap(child);
+  const canWrap = getWrap(child);
+
+  // Calculate the y coordinate where the desired presence of the child ends
+  const endOfPresence = getEndOfPresence(child, futureElements);
+  // If the child is already at the top of the page, breaking won't improve its presence
+  // (as long as react-pdf does not support breaking into differently sized containers)
+  const breakingImprovesPresence = child.box.top > child.box.marginTop;
 
   return (
     getBreak(child) ||
-    (!shouldWrap && shouldSplit) ||
-    (minPresenceAhead < futureHeight && presenceAhead < minPresenceAhead)
+    (shouldSplit && !canWrap) ||
+    (!shouldSplit && endOfPresence > height && breakingImprovesPresence)
   );
 };