feat: select deepest candidate when using nested droppables

src/state/droppable/get-droppable.ts

@@ -32,6 +32,7 @@ interface Args {
   page: BoxModel;
   closest?: Closest | null;
   transform: Transform | null;
+  parents: DroppableDescriptor[];
 }
 
 export default ({
@@ -44,6 +45,7 @@ export default ({
   page,
   closest,
   transform,
+  parents,
 }: Args): DroppableDimension => {
   const frame: Scrollable | null = (() => {
     if (!closest) {
@@ -98,6 +100,7 @@ export default ({
     frame,
     subject,
     transform,
+    parents,
   };
 
   return dimension;

src/state/get-droppable-over.ts

@@ -75,6 +75,67 @@ function getFurthestAway({
   return sorted[0] ? sorted[0].id : null;
 }
 
+/**
+ * normalizeFamilies
+ *
+ * Groups all items that share a common root `parent`, and selects the deepest item
+ * in that group that contains the center point of the dragged item to represent
+ * the "family".
+ */
+function normalizeFamilies(
+  pageBorderBox: Rect,
+  candidates: DroppableDimension[],
+) {
+  const families = candidates.reduce<Record<string, DroppableDimension[]>>(
+    (acc, candidate) => {
+      const familyName = candidate.parents[0]?.id || candidate.descriptor.id;
+      const family = acc[familyName] || [];
+
+      const generation = candidate.parents.length;
+
+      let chosenCandidateId: string | null = candidate.descriptor.id;
+
+      if (family[generation]) {
+        // Overlapping with 2 items at the same generation. get furthest away.
+
+        chosenCandidateId = getFurthestAway({
+          pageBorderBox,
+          draggable,
+          candidates: [candidate, family[generation]],
+        });
+      }
+
+      if (chosenCandidateId) {
+        family[generation] = candidate;
+      }
+
+      return {
+        ...acc,
+        [familyName]: family,
+      };
+    },
+    {},
+  );
+
+  return Object.keys(families).map((familyName) => {
+    const family = families[familyName];
+
+    const reversedFamily = [...family].reverse();
+
+    // Get first member of family that contains the draggable
+    const chosenMember = reversedFamily.find((member) => {
+      return (
+        pageBorderBox.center.x < member.page.borderBox.right &&
+        pageBorderBox.center.x > member.page.borderBox.left &&
+        pageBorderBox.center.y > member.page.borderBox.top &&
+        pageBorderBox.center.y < member.page.borderBox.bottom
+      );
+    });
+
+    return chosenMember || family[0];
+  });
+}
+
 export default function getDroppableOver({
   pageBorderBox,
   draggable,
@@ -146,12 +207,19 @@ export default function getDroppableOver({
     return candidates[0].descriptor.id;
   }
 
-  // Multiple options returned
+  // Select the best candidate from each group that share a common root ancestor
+  const normalizedCandidates = normalizeFamilies(pageBorderBox, candidates);
+
+  // All candidates were in the same family
+  if (normalizedCandidates.length === 1) {
+    return normalizedCandidates[0].descriptor.id;
+  }
+
   // Should only occur with really large items
   // Going to use fallback: distance from home
   return getFurthestAway({
     pageBorderBox,
     draggable,
-    candidates,
+    candidates: normalizedCandidates,
   });
 }

src/types.ts

@@ -154,6 +154,7 @@ export interface DroppableDimension {
   // what is visible through the frame
   subject: DroppableSubject;
   transform: Transform | null;
+  parents: DroppableDescriptor[];
 }
 export interface DraggableLocation {
   droppableId: DroppableId;

src/view/use-droppable-publisher/get-dimension.ts

@@ -14,6 +14,7 @@ import getIframeOffset from '../iframe/get-iframe-offset';
 import { applyOffsetBox } from '../iframe/apply-offset';
 import { Transform, applyTransformBox, getTransform } from '../transform';
 import { Offset } from '../iframe/offset-types';
+import { prefix } from '../data-attributes';
 
 const getClient = (
   targetRef: HTMLElement,
@@ -96,6 +97,34 @@ interface Args {
   transform: Transform | null;
 }
 
+const getParents = (ref: HTMLElement) => {
+  const contextId = ref.getAttribute(`${prefix}-droppable-context-id`);
+
+  const parentDescriptors: DroppableDescriptor[] = [];
+
+  if (!contextId) return [];
+
+  let currentEl: HTMLElement | null | undefined = ref;
+
+  while (currentEl) {
+    currentEl = currentEl.parentElement?.closest(
+      `[${prefix}-droppable-context-id="${contextId}"]`,
+    );
+
+    const id = currentEl?.getAttribute(`${prefix}-droppable-id`);
+
+    if (id) {
+      parentDescriptors.push({
+        id,
+        mode: 'standard',
+        type: 'DEFAULT',
+      });
+    }
+  }
+
+  return parentDescriptors;
+};
+
 export default ({
   ref,
   descriptor,
@@ -132,6 +161,8 @@ export default ({
     };
   })();
 
+  const parents = getParents(ref);
+
   const dimension: DroppableDimension = getDroppableDimension({
     descriptor,
     isEnabled: !isDropDisabled,
@@ -142,6 +173,7 @@ export default ({
     page,
     closest,
     transform,
+    parents,
   });
 
   return dimension;