bug: ion-datetime walks workingParts back a month when the calendar body has no layout #31416

Prerequisites

• I have read the Contributing Guidelines.
• I agree to follow the Code of Conduct.
• I have searched for existing issues that already report this problem.

Ionic Framework Version

v8.8.19 (also present in 8.8.9; unaffected by #31335 in 8.8.18)

Current Behavior

initializeCalendarListener's scroll handler infers the displayed month from the calendar body's
geometry, and getChangedMonth runs even when the body has no layout at all — while
:host(.show-month-and-year) .calendar-body { display: none } applies, or while the host overlay is
hidden. Every box then measures 0, and the handler's own test

const startMonth = (isRTL(this.el) ? calendarBodyRef.scrollLeft >= -2 : calendarBodyRef.scrollLeft <= 2)
  ? months[0] : months[2];

reads scrollLeft === 0 as "the user has scrolled to the previous month". Math.abs(rect.x - box.x) > 2
does not filter it out either, because with no layout both rects are zero and the difference is 0.

So merely opening the month/year wheel can move workingParts back a month. It is user-visible
immediately: the wheel columns are bound to workingParts, so the month the wheel displays drifts
backwards from the one the user was looking at, and any subsequent commit uses the drifted value.

The 50 ms scroll debounce is what makes this intermittent rather than constant — measured in a
headless Chrome harness, it reproduces in roughly four runs out of five.

Expected Behavior

A calendar body with no layout has no meaningful scroll position, so it should not drive a month
change. Hiding and re-showing the grid should leave workingParts untouched.

Steps to Reproduce

1. Render a bare datetime: <ion-datetime presentation="date-time" value="2026-04-28T10:00:00"></ion-datetime>
2. Tap the previous month arrow a few times — say until the header reads November 2025.
3. Tap the header to open the month/year wheel.
4. Wait ~300 ms and read the month column / header.

Actual: the month drifts backwards (November → October, and further on repeat opens).
Expected: it stays on November 2025.

Repeat a few times if it doesn't show on the first attempt — see the debounce note above.

Additional Information

A guard on the first statement of getChangedMonth fixes it:

const getChangedMonth = (parts: DatetimeParts): DatetimeParts | undefined => {
  if (calendarBodyRef.clientWidth === 0 && this.forceRenderDate === undefined) return;
  // ...

The forceRenderDate half is not defensive noise, and an unconditional early return is a trap.
animateToDate sets forceRenderDate, awaits a promise that only updateActiveMonth resolves, and
clears forceRenderDate afterwards. Returning early unconditionally strands that promise for the
lifetime of the component: forceRenderDate never clears, so every later scroll takes
getChangedMonth's forced-date branch and commits a stale date. Reading this.forceRenderDate at
callback time (not destructuring it alongside calendarBodyRef/months at listener-init time) is
also required, or the guard captures undefined once and is permanently true.

We carry this as a patch-package patch locally and would be happy to open a PR.

Related: the calendar header and the day grid can also disagree for a different reason — the header
renders from workingParts synchronously while the 3-pane carousel is re-centred only by that same
debounced listener, so tapping the arrows faster than it settles leaves them out of sync. Separate
from this issue; happy to file it if useful.