In new quick-action-panel cmp I am using spinner as this. slds-is-relative will remove the ugly rectangular shadow and thanks to style="min-height: 50vh" the spinner will be visible whole.
<lightning-quick-action-panel header="asd"> <div class="slds-is-relative" style="min-height: 50vh" if:true={spinner}> <lightning-spinner variant="brand"></lightning-spinner> </div> </lightning-quick-action-panel>