I struggled for a while trying to build a dependent select filter, trying to do it as two separate filters. Turns out the trick is to build it as a single form based filter, with both selects in the custom form.

In my app, we have a FlightLegs table, which pilots use to log flight hours for missions flown in an aircraft for a given day. FlightLegs belong to a DailyLog table, which stores the overall mission/aircraft details - aircraft, flight date, crew members, etc. Aircraft belong to Vendors (contracted aircraft operators). So from the FlightLegs table, the relationship to Aircraft is dailyLog.aircraft, and aircraft belongsTo Vendor.

I needed a pair of filters on the FlightLeg table, filtering for the Vendor and Aircraft, with Aircraft being dependent on Vendor - so if a vendor is selected, only show the aircraft for that vendor, and when a Vendor is selected, clear any current Aircraft selection if it doesn't belong to the selected Vendor.

// filter name can be anything you want
SelectFilter::make('aircraft')
    ->label('Aircraft')
    // build the filter form
    ->form([
        // build the Vendor dropdown
        Select::make('vendor_id')
            ->label('Vendor')
            // select all Vendors
            ->options(
                fn() => Vendor::all()->pluck('name', 'id')->toArray()
            )
            // when state updated, check to see if the currently selected aircraft belongs to the selected vendor ...
            ->afterStateUpdated(function ($state, callable $get, callable $set) {
                $vendor = Vendor::find($state);
 
                if ($vendor) {
                    $aircraftId = (int) $get('aircraft_id');
 
                    if ($aircraftId && $aircraft = Aircraft::find($aircraftId)) {
                        if ($aircraft->vendor_id !== $vendor->id) {
                            // aircraft doesn't belong to vendor, so unselect it
                            $set('aircraft_id', null);
                        }
                    }
                }
            })
            // make the vendor dropdown reactive so we can rebuild the aircraft dropdown when state changes
            ->reactive(),
        // build the Aircraft dropdown
        Select::make('aircraft_id')
            ->label('Aircraft')
            // build the options
            ->options(function (callable $get, callable $set) {
                $vendor = Vendor::find($get('vendor_id'));
 
                // if a vendor is selected, just fetch the aircraft for this vendor thru the vendor's belongsTo aircraft
                if ($vendor) {
                    return $vendor->aircraft->pluck('n_number', 'id');
                }
 
                // no vendor selected, so get all aircraft
                return Aircraft::all()->pluck('n_number', 'id');
            }),
    ])
    // handle the query
    ->query(function (Builder $query, array $data) {
        // get the vendor and aircraft id's from the form's $data array
        $aircraftId = (int) $data['aircraft_id'];
        $vendorId   = (int) $data['vendor_id'];
 
        // if an aircraft selected, scope the query to just that aircraft id with a nested whereHas
        if (!empty($aircraftId)) {
            $query->whereHas(
                'dailyLog',
                fn(Builder $query) => $query->whereHas(
                    'aircraft',
                    fn(Builder $query) => $query->where('id', '=', $aircraftId)
                )
            );
        }
 
        // if a vendor selected, scope the query to just aircraft with that vendor id with a nested whereHas
        if (!empty($vendorId)) {
            $query->whereHas(
                'dailyLog',
                fn(Builder $query) => $query->whereHas(
                    'aircraft',
                    fn(Builder $query) => $query->where('vendor_id', '=', $vendorId)
                )
            );
        }
 
    }),