ionic-framework/core/src/components/content/content.tsx

import { Component, Element, Listen, Method, Prop, QueueApi } from '@stencil/core';

import { Color, Config, Mode } from '../../interface';
import { createColorClasses } from '../../utils/theme';

@Component({
  tag: 'ion-content',
  styleUrls: {
    ios: 'content.ios.scss',
    md: 'content.md.scss'
  },
  shadow: true
})
export class Content {

  private cTop = -1;
  private cBottom = -1;
  private scrollEl?: HTMLIonScrollElement;

  mode!: Mode;
  @Prop() color?: Color;

  @Element() el!: HTMLStencilElement;

  @Prop({ context: 'config' }) config!: Config;
  @Prop({ context: 'queue' }) queue!: QueueApi;

  /**
   * If true, the content will scroll behind the headers
   * and footers. This effect can easily be seen by setting the toolbar
   * to transparent.
   */
  @Prop() fullscreen = false;

  /**
   * If true and the content does not cause an overflow scroll, the scroll interaction will cause a bounce.
   * If the content exceeds the bounds of ionContent, nothing will change.
   * Note, the does not disable the system bounce on iOS. That is an OS level setting.
   */
  @Prop() forceOverscroll?: boolean;

  /**
   * By default `ion-content` uses an `ion-scroll` under the hood to implement scrolling,
   * if you want to disable the content scrolling for a given page, set this property to `false`.
   */
  @Prop() scrollEnabled = true;

  /**
   * Because of performance reasons, ionScroll events are disabled by default, in order to enable them
   * and start listening from (ionScroll), set this property to `true`.
   */
  @Prop() scrollEvents = false;

  @Listen('body:ionNavDidChange')
  onNavChanged() {
    this.resize();
  }

  componentDidLoad() {
    this.resize();
  }

  @Method()
  getScrollElement(): HTMLIonScrollElement {
    return this.scrollEl!;
  }

  private resize() {
    if (!this.scrollEl) {
      return;
    }
    if (this.fullscreen) {
      this.queue.read(this.readDimensions.bind(this));
    } else if (this.cTop !== 0 || this.cBottom !== 0) {
      this.cTop = this.cBottom = 0;
      this.el.forceUpdate();
    }
  }

  private readDimensions() {
    const page = getPageElement(this.el);
    const top = Math.max(this.el.offsetTop, 0);
    const bottom = Math.max(page.offsetHeight - top - this.el.offsetHeight, 0);
    const dirty = top !== this.cTop || bottom !== this.cBottom;
    if (dirty) {
      this.cTop = top;
      this.cBottom = bottom;
      this.el.forceUpdate();
    }
  }

  hostData() {
    return {
      class: {
        ...createColorClasses(this.color),
        'scroll-disabled': !this.scrollEnabled,
      }
    };
  }

  render() {
    this.resize();

    return [
      this.scrollEnabled ? (
        <ion-scroll
          ref={el => this.scrollEl = el as any}
          mode={this.mode}
          scrollEvents={this.scrollEvents}
          forceOverscroll={this.forceOverscroll}
          style={{
            'top': `${-this.cTop}px`,
            'bottom': `${-this.cBottom}px`,
            '--offset-top': `${this.cTop}px`,
            '--offset-bottom': `${this.cBottom}px`,
          }}>
            <slot></slot>
        </ion-scroll>
      ) : <slot></slot>,
      <slot name="fixed"></slot>
    ];
  }
}

function getParentElement(el: any) {
  if (el.parentElement) {
    // normal element with a parent element
    return el.parentElement;
  }
  if (el.parentNode && el.parentNode.host) {
    // shadow dom's document fragment
    return el.parentNode.host;
  }
  return null;
}

function getPageElement(el: HTMLElement) {
  const tabs = el.closest('ion-tabs');
  if (tabs) {
    return tabs;
  }
  const page = el.closest('ion-app,ion-page,.ion-page,page-inner');
  if (page) {
    return page;
  }
  return getParentElement(el);
}