Closes #22754

Root cause

When dragging an external event (StandaloneEvent) into the Event Calendar, if the pointer leaves the drop surface and then re-enters at the same position, the drag placeholder stayed invisible. It only reappeared once the pointer moved to a different slot.

shouldUpdateOccurrencePlaceholder in packages/x-scheduler-internals/src/internals/utils/SchedulerStore/SchedulerStore.utils.ts is the dirty-check that decides whether setOccurrencePlaceholder writes a new placeholder to the store. Its diff was asymmetric: it iterated only over the keys of next, so a key that exists on previous but is absent on next was never compared.

The exact flow that triggers it:

1. onDragLeave sets the placeholder to { ...current, isHidden: true }. For external drag this always happens, and the selectors hide the placeholder while isHidden is truthy.
2. On re-enter, onDrag builds a fresh placeholder that does not include isHidden.
3. shouldUpdateOccurrencePlaceholder(previous = { ..., isHidden: true }, next = { ... no isHidden }) loops only over next's keys, so the isHidden key (present on previous, missing on next) is never inspected. With everything else identical, it returns false, the store is not updated, and the placeholder stays hidden.

This was not specific to isHidden. Any optional key present on previous but dropped from next was ignored by the diff.

Change

Make the diff symmetric. After the existing pass over next's keys, a second pass over previous's keys returns true as soon as it finds a key that no longer exists on next:

// Catch keys present in `previous` but removed from `next` (e.g. `isHidden`).
for (const key in untypedPrevious) {
  if (!(key in untypedNext)) {
    return true;
  }
}

start and end are required on every placeholder, so they never appear in the second pass and the adapter.isEqual special-case is untouched. The pass is allocation-free and keeps the early-exit, so the drag hot path is unaffected.

Why it is safe

Every placeholder construction site was reviewed. No production flow relies on the previous asymmetric behavior. The only present-in-previous and absent-in-next transition that happens with everything else unchanged is the isHidden un-hide case (the bug). The other optional keys (lockSurfaceType, onEventDrop) only change alongside a required key that is already present on next, so the diff result is identical there.

Tests

Added tests in SchedulerStore.utils.test.ts covering shouldUpdateOccurrencePlaceholder, including the regression case: previous with isHidden: true versus an otherwise identical next without isHidden must return true. The regression test fails on the original code and passes with the fix. Typecheck, prettier, and eslint are clean.

How to reproduce and verify

In the External drag and drop demo on the Event Calendar drag interactions page:

1. Drag the external event into the calendar and hover a column so the placeholder appears.
2. Drag out of the grid so the placeholder hides.
3. Drag back into the same column at the same position.

Before this change the placeholder stays hidden until you move to another slot. After this change it reappears immediately.

• I have followed (at least) the PR section of the contributing guide.