How to use the @deckdeckgo/utils.isFullscreen function in @deckdeckgo/utils

private onWindowResize = async () => {
        this.fullscreen = isFullscreen();

        // Per default, when we switch to the fullscreen mode, we want to present the presentation not edit it
        await this.updatePresenting(this.fullscreen);
    };

await this.fetchSlides();
            }

            this.slidesFetched = true;
        });

        this.busySubscription = this.busyService.watchSlideEditable().subscribe(async (slide: HTMLElement) => {
            // Hide actions footer till deck is editable
            this.hideFooterActions = false;

            this.slidesEditable = true;

            await this.contentEditable(slide);
        });

        this.fullscreen = isFullscreen();
    }

async inactivity($event: CustomEvent) {
        if (!isFullscreen()) {
            return;
        }

        this.displayed = $event.detail;

        await this.blurSelectedElement();
    }

window.addEventListener('resize', debounce(async () => {
        await this.initSlideSize();
        await this.slideTo(this.activeIndex);

        const toggleFullscreen: boolean = isFullscreen();
        await this.hideOrClearMouseCursorTimer(toggleFullscreen);
        await this.showHideActionsSlot(toggleFullscreen);
      }, 100));
    }