* fix: form state now correctly updates members when hidden state cha…

…nges (#3719)

Underlying issue: immutableReconcile did not account for changes in number of keys in objects,
which resulted in it considering next object unchanged when a key had been removed.
This made the code retain the previous value incorrectly,

This had the effect of associating wrong schemaTypes with member fields, when hidden function
changed state (as one observed effect).

packages/sanity/src/core/form/store/utils/__tests__/immutableReconcile.test.ts

@@ -87,3 +87,27 @@ test('does not mutate any of its input', () => {
 
   expect(() => immutableReconcile(prev, next)).not.toThrow()
 })
+
+test('returns new object when previous and next has different number of keys', () => {
+  const moreKeys = {
+    key1: 'value1',
+    key2: 'value2',
+    key3: 'value3',
+  }
+  const lessKeys = {
+    key1: 'value1',
+    key2: 'value2',
+  }
+
+  expect(immutableReconcile(moreKeys, lessKeys)).not.toBe(moreKeys)
+  expect(immutableReconcile(lessKeys, moreKeys)).not.toBe(lessKeys)
+})
+
+test('returns new array when previous and next has different length', () => {
+  const moreItems = ['a', 'b']
+  const lessItems = ['a']
+
+  expect(immutableReconcile(moreItems, lessItems)).not.toBe(moreItems)
+
+  expect(immutableReconcile(lessItems, moreItems)).not.toBe(lessItems)
+})

packages/sanity/src/core/form/store/utils/immutableReconcile.ts

@@ -55,9 +55,10 @@ function _immutableReconcile<T>(
     assertType<Record<string, unknown>>(previous)
     assertType<Record<string, unknown>>(next)
 
-    let allEqual = true
+    const nextKeys = Object.keys(next)
+    let allEqual = Object.keys(previous).length === nextKeys.length
     const result: Record<string, unknown> = {}
-    for (const key of Object.keys(next)) {
+    for (const key of nextKeys) {
       if (parents.has(next[key])) {
         return next
       }