ngbPopoverClose.ts

import {
    ComponentRef,
    Directive,
    ElementRef,
    Input,
    OnDestroy,
    OnInit
} from '@angular/core';
import { filter, takeUntil } from 'rxjs/operators';
import { fromEvent } from 'rxjs/observable/fromEvent';
import { merge } from 'rxjs/observable/merge';
import { NgbPopover } from '@ng-bootstrap/ng-bootstrap';
import { NgbPopoverWindow } from '@ng-bootstrap/ng-bootstrap/popover/popover';
import { Subject } from 'rxjs/Subject';

@Directive({
    selector: '[handlePopoverClose][ngbPopover]'
})
export class HandlePopoverCloseDirective implements OnInit, OnDestroy {

    @Input('handlePopoverClose') closeOnPopoverBodyClick = false;

    private destroy$ = new Subject<void>();

    constructor(
        private elementRef: ElementRef,
        private ngbPopover: NgbPopover) {
    }

    ngOnInit() {
        const closedOrDestroyed$ = merge(this.ngbPopover.hidden, this.destroy$);
        const events$ = merge(
            fromEvent(document, 'click'),
            fromEvent(document, 'keydown').pipe(filter((event: KeyboardEvent) => event.keyCode === 27)) // esc
        );

        this.ngbPopover.shown.subscribe(() => {
            events$
                .pipe(takeUntil(closedOrDestroyed$))
                .subscribe((event: MouseEvent) => this.handleCloseEvents(event));
        });
    }

    ngOnDestroy() {
        this.destroy$.next();
        this.destroy$.complete();
    }

    private handleCloseEvents(event: MouseEvent): void {
        if (event.type === 'keydown') {
            this.ngbPopover.close();
            return;
        }

        const target = event.target as Element;
        if (!this.elementRef.nativeElement.contains(target)) {
            const popoverWindowRef: ComponentRef<NgbPopoverWindow> = (this.ngbPopover as any)._windowRef;
            if (!popoverWindowRef.location.nativeElement.contains(event.target) || this.closeOnPopoverBodyClick) {
                this.ngbPopover.close();
            }
        }
    }
}

If you wanna use this directive and you're using RxJs 6, it's necessary to update the syntax for importing the packages like that:

import { Subject, fromEvent, merge } from 'rxjs';
import { filter, takeUntil } from 'rxjs/operators';