Summary

This PR ensures that only the necessary pieces of layout state are backed up when a dashboard has unsaved changes. Specifically, it fixes two separate bugs with backup state:

1. Consider the following code:

   kibana/src/platform/plugins/shared/dashboard/public/dashboard_api/unsaved_changes_manager.ts

   Lines 125 to 128 in 18f8010

   	if (hasChildrenUnsavedChanges) {
   	dashboardBackupState.panels = panels;
   	dashboardBackupState.controlGroupInput = controlGroupInput;
   	}

   Because [Controls Anywhere] Feature Branch #245588 made pinned controls normal children of the Dashboard rather than having separate unsaved changes management through the control group, the above code resulted in panels being backed up as unsaved changes even if only controls had changes. For example, making a single selection in a control resulted in both controls and panels being backed up to session storage - which caused extremely long URLs when Short URL permissions were turned off.

2. Now consider the following:

   kibana/src/platform/plugins/shared/dashboard/public/dashboard_api/unsaved_changes_manager.ts

   Lines 83 to 88 in 18f8010

   	const dashboardStateChanges$ = combineLatest([
   	settingsManager.internalApi.startComparing$(lastSavedState$),
   	unifiedSearchManager.internalApi.startComparing$(lastSavedState$),
   	layoutManager.internalApi.startComparing$(lastSavedState$),
   	projectRoutingManager?.internalApi.startComparing$(lastSavedState$) ?? of({}),
   	]).pipe(

   kibana/src/platform/plugins/shared/dashboard/public/dashboard_api/layout_manager/layout_manager.ts

   Lines 492 to 517 in 18f8010

   	startComparing$: (
   	lastSavedState$: BehaviorSubject<DashboardState>
   	): Observable<{ panels?: DashboardState['panels'] }> => {
   	return layout$.pipe(
   	debounceTime(100),
   	combineLatestWith(
   	lastSavedState$.pipe(
   	map((lastSaved) =>
   	deserializeLayout(lastSaved.panels, lastSaved.controlGroupInput, getReferences)
   	),
   	tap(({ layout, childState }) => {
   	lastSavedChildState = childState;
   	lastSavedLayout = layout;
   	})
   	)
   	),
   	map(([currentLayout]) => {
   	if (!areLayoutsEqual(lastSavedLayout, currentLayout)) {
   	logStateDiff('dashboard layout', lastSavedLayout, currentLayout);
   	return serializeLayout(currentLayout, currentChildState);
   	}
   	return {};
   	})
   	);
   	},

   Because we were using isLayoutEqual, which is checking for layout changes to both controls and panel layouts, we were reporting layout changes for panels even if only controls changed. For example, reordering a single control resulted in both controls and panels being backed up to session storage - which caused extremely long URLs when Short URL permissions were turned off.

This PR fixes both of these bugs by...

1. We now compare which children actually changed and determine which ones are controls and which are normal panels. We only backup panels if at least one panel child changed, and same with controls.

2. We now separate out our comparison into arePanelLayoutsEqual and areControlsLayoutsEqual and we conditionally return each piece of state as unsaved depending on the results.

Checklist

• Unit or functional tests were updated or added to match the most common scenarios
• The PR description includes the appropriate Release Notes section, and the correct release_note:* label is applied per the guidelines
• Review the backport guidelines and apply applicable backport:* labels.