Closes #190066

Summary

In #189128 (comment), I suggested adding a skip to the comparators subscription in order to prevent unsavedChanges from emitting twice - however, while I was on the right track with this suggestion, I didn't look closely enough at how this worked. Essentially, this is how it worked without the skip when creating a brand new React embeddable:

1. We initialize unsavedChanges by calling runComparators to get its initial value - since it is a new embeddable and therefore does not have unsaved state, it returns the entire state per the early return in runComparators
2. The comparator subjects all emit and, after the debounce, unsavedChanges gets set again by calling runComparators - similarly, it returns the entire state due to the early return.
3. Unsaved changes emits twice unnecessarily - once at initialization, once after a debounce 🔥

By adding the skip where I suggested, this is what happened:

1. We initialize unsavedChanges by calling runComparators to get its initial value - it returns the entire state due to the early return.
2. We ignore the first emit of every comparator due to the skip - this is good, because unsaved changes emits only once!
3. But, when we go to save the dashboard, the placement of the skip made it so that the subscription did not fire when lastSavedState emits - it is waiting for the comparators to emit again before firing! So, the unsaved changes remains unnecessarily 🔥

Essentially, we want to skip the first emit of the whole comparator + lastSavedState observable that results from the pipe - not just the comparators. Otherwise, we don't get to the combineLatestWith until after the comparators emit a second time, which means we don't respond whenlastSavedState emits after a React embeddable panel is freshly added. By rearranging the order of operations in the pipe, we achieve the desired behaviour.

To test the above, move the skip to before the lastSavedState emit. Add a React embeddable panel, and try to save - unsaved changes will be stuck. On a fresh dashboard, add a React embeddable panel again, edit that React embeddable to trigger a comparator emit, then save - unsaved changes is cleared!

Checklist

• Unit or functional tests were updated or added to match the most common scenarios

For maintainers

• This was checked for breaking API changes and was labeled appropriately