From f69210b6d184336f8a57c288f2dca3d7db2733eb Mon Sep 17 00:00:00 2001
From: Jasper Vriends <4417659+jaspervriends@users.noreply.github.com>
Date: Mon, 8 Aug 2022 14:29:14 +0200
Subject: [PATCH] feat(modals): support stacking modals, remove bootstrap
 modals dependency (#3456)

* Feature: Stackable modals

* Processed feedback

* fix: use position in modal stack rather than key for modal number

* fix: use correct base z-index

* chore: simplify `className` to `class`

* chore: add `key` attribute to ModalManager element

* fix: backdrop flashing as modals are stacked/unstacked

* chore: simplify modal close process

* docs: add TS overload to indicate deprecated modal opening syntax

Require explicit values for `attrs` and `stackModal` from Flarum 2.0, beginning deprecation from now.

* feat: use stackable modal for forgot password modal above sign in

* chore: explicitly check if modal is open before trying to focus trap

* fix(a11y): add missing  `aria-hidden` on main content when modal open

* fix(a11y): add missing `aria-modal` on modal

* chore: remove test code

* chore: remove dead CSS

* chore: remove overload

* fix: lock page scrolling when modal is open

* fix: strange scrolling behaviour

* chore: convert to JSX

* fix: event listener memory leak

* chore: remove unneeded optional chaining

* fix: incorrect return types

* chore: rewrite backdrop system - use one backdrop for all modals

* docs: typos in comment block

* fix: show backdrop directly below top-most modal

Co-authored-by: Sami Mazouz <sychocouldy@gmail.com>

* chore: format

* fix: use an invisible backdrop for each modal to exit

Signed-off-by: Sami Mazouz <ilyasmazouz@gmail.com>

* chore: remove debugging code

Signed-off-by: Sami Mazouz <ilyasmazouz@gmail.com>

* chore: remove forgotten debug code

Co-authored-by: David Wheatley <david@davwheat.dev>
Co-authored-by: Sami Mazouz <sychocouldy@gmail.com>
---
 framework/core/js/package.json                |   2 +
 framework/core/js/src/common/Application.tsx  |   4 +-
 .../core/js/src/common/components/Modal.tsx   |  71 +++++-
 .../js/src/common/components/ModalManager.tsx | 217 ++++++++++++++----
 framework/core/js/src/common/index.ts         |   1 -
 .../js/src/common/states/ModalManagerState.ts |  88 +++++--
 framework/core/less/common/Modal.less         | 102 ++++----
 yarn.lock                                     |  10 +
 8 files changed, 357 insertions(+), 138 deletions(-)

diff --git a/framework/core/js/package.json b/framework/core/js/package.json
index 259d242ea..1a7a565c7 100644
--- a/framework/core/js/package.json
+++ b/framework/core/js/package.json
@@ -6,6 +6,7 @@
     "dependencies": {
         "@askvortsov/rich-icu-message-formatter": "^0.2.4",
         "@ultraq/icu-message-formatter": "^0.12.0",
+        "body-scroll-lock": "^4.0.0-beta.0",
         "bootstrap": "^3.4.1",
         "clsx": "^1.1.1",
         "color-thief-browser": "^2.0.2",
@@ -21,6 +22,7 @@
     },
     "devDependencies": {
         "@flarum/prettier-config": "^1.0.0",
+        "@types/body-scroll-lock": "^3.1.0",
         "@types/jquery": "^3.5.10",
         "@types/mithril": "^2.0.8",
         "@types/punycode": "^2.1.0",
diff --git a/framework/core/js/src/common/Application.tsx b/framework/core/js/src/common/Application.tsx
index 29c6ea80d..5db7fa84e 100644
--- a/framework/core/js/src/common/Application.tsx
+++ b/framework/core/js/src/common/Application.tsx
@@ -319,8 +319,8 @@ export default class Application {
 
   protected mount(basePath: string = '') {
     // An object with a callable view property is used in order to pass arguments to the component; see https://mithril.js.org/mount.html
-    m.mount(document.getElementById('modal')!, { view: () => ModalManager.component({ state: this.modal }) });
-    m.mount(document.getElementById('alerts')!, { view: () => AlertManager.component({ state: this.alerts }) });
+    m.mount(document.getElementById('modal')!, { view: () => <ModalManager state={this.modal} /> });
+    m.mount(document.getElementById('alerts')!, { view: () => <AlertManager state={this.alerts} /> });
 
     this.drawer = new Drawer();
 
diff --git a/framework/core/js/src/common/components/Modal.tsx b/framework/core/js/src/common/components/Modal.tsx
index b8656860d..4cde799b0 100644
--- a/framework/core/js/src/common/components/Modal.tsx
+++ b/framework/core/js/src/common/components/Modal.tsx
@@ -8,6 +8,7 @@ import type ModalManagerState from '../states/ModalManagerState';
 import type RequestError from '../utils/RequestError';
 import type ModalManager from './ModalManager';
 import fireDebugWarning from '../helpers/fireDebugWarning';
+import classList from '../utils/classList';
 
 export interface IInternalModalAttrs {
   state: ModalManagerState;
@@ -15,16 +16,63 @@ export interface IInternalModalAttrs {
   animateHide: ModalManager['animateHide'];
 }
 
+export interface IDismissibleOptions {
+  /**
+   * @deprecated Check specific individual attributes instead. Will be removed in Flarum 2.0.
+   */
+  isDismissible: boolean;
+  viaCloseButton: boolean;
+  viaEscKey: boolean;
+  viaBackdropClick: boolean;
+}
+
 /**
  * The `Modal` component displays a modal dialog, wrapped in a form. Subclasses
  * should implement the `className`, `title`, and `content` methods.
  */
 export default abstract class Modal<ModalAttrs extends IInternalModalAttrs = IInternalModalAttrs> extends Component<ModalAttrs> {
+  // TODO: [Flarum 2.0] remove `isDismissible` static attribute
   /**
    * Determine whether or not the modal should be dismissible via an 'x' button.
+   *
+   * @deprecated Use the individual `isDismissibleVia...` attributes instead and remove references to this.
    */
   static readonly isDismissible: boolean = true;
 
+  /**
+   * Can the model be dismissed with a close button (X)?
+   *
+   * If `false`, no close button is shown.
+   */
+  protected static readonly isDismissibleViaCloseButton: boolean = true;
+  /**
+   * Can the modal be dismissed by pressing the Esc key on a keyboard?
+   */
+  protected static readonly isDismissibleViaEscKey: boolean = true;
+  /**
+   * Can the modal be dismissed via a click on the backdrop.
+   */
+  protected static readonly isDismissibleViaBackdropClick: boolean = true;
+
+  static get dismissibleOptions(): IDismissibleOptions {
+    // If someone sets this to `false`, provide the same behaviour as previous versions of Flarum.
+    if (!this.isDismissible) {
+      return {
+        isDismissible: false,
+        viaCloseButton: false,
+        viaEscKey: false,
+        viaBackdropClick: false,
+      };
+    }
+
+    return {
+      isDismissible: true,
+      viaCloseButton: this.isDismissibleViaCloseButton,
+      viaEscKey: this.isDismissibleViaEscKey,
+      viaBackdropClick: this.isDismissibleViaBackdropClick,
+    };
+  }
+
   protected loading: boolean = false;
 
   /**
@@ -70,7 +118,6 @@ export default abstract class Modal<ModalAttrs extends IInternalModalAttrs = IIn
     // we've just opened up a new one, and accordingly,
     // we don't need to show a hide animation.
     if (!this.attrs.state.modal) {
-      this.attrs.animateHide();
       // Here, we ensure that the animation has time to complete.
       // See https://mithril.js.org/lifecycle-methods.html#onbeforeremove
       // Bootstrap's Modal.TRANSITION_DURATION is 300 ms.
@@ -87,16 +134,16 @@ export default abstract class Modal<ModalAttrs extends IInternalModalAttrs = IIn
     }
 
     return (
-      <div className={'Modal modal-dialog ' + this.className()}>
+      <div className={classList('Modal modal-dialog fade', this.className())}>
         <div className="Modal-content">
-          {(this.constructor as typeof Modal).isDismissible && (
+          {this.dismissibleOptions.viaCloseButton && (
             <div className="Modal-close App-backControl">
-              {Button.component({
-                icon: 'fas fa-times',
-                onclick: () => this.hide(),
-                className: 'Button Button--icon Button--link',
-                'aria-label': app.translator.trans('core.lib.modal.close'),
-              })}
+              <Button
+                icon="fas fa-times"
+                onclick={() => this.hide()}
+                className="Button Button--icon Button--link"
+                aria-label={app.translator.trans('core.lib.modal.close')}
+              />
             </div>
           )}
 
@@ -149,7 +196,7 @@ export default abstract class Modal<ModalAttrs extends IInternalModalAttrs = IIn
    * Hides the modal.
    */
   hide(): void {
-    this.attrs.state.close();
+    this.attrs.animateHide();
   }
 
   /**
@@ -175,4 +222,8 @@ export default abstract class Modal<ModalAttrs extends IInternalModalAttrs = IIn
       this.onready();
     }
   }
+
+  private get dismissibleOptions(): IDismissibleOptions {
+    return (this.constructor as typeof Modal).dismissibleOptions;
+  }
 }
diff --git a/framework/core/js/src/common/components/ModalManager.tsx b/framework/core/js/src/common/components/ModalManager.tsx
index 454ae4818..a8850dfea 100644
--- a/framework/core/js/src/common/components/ModalManager.tsx
+++ b/framework/core/js/src/common/components/ModalManager.tsx
@@ -2,6 +2,8 @@ import Component from '../Component';
 
 import { createFocusTrap, FocusTrap } from '../utils/focusTrap';
 
+import { disableBodyScroll, clearAllBodyScrollLocks } from 'body-scroll-lock';
+
 import type ModalManagerState from '../states/ModalManagerState';
 import type Mithril from 'mithril';
 
@@ -10,46 +12,81 @@ interface IModalManagerAttrs {
 }
 
 /**
- * The `ModalManager` component manages a modal dialog. Only one modal dialog
- * can be shown at once; loading a new component into the ModalManager will
- * overwrite the previous one.
+ * The `ModalManager` component manages one or more modal dialogs. Stacking modals
+ * is supported. Multiple dialogs can be shown at once; loading a new component
+ * into the ModalManager will overwrite the previous one.
  */
 export default class ModalManager extends Component<IModalManagerAttrs> {
+  // Current focus trap
   protected focusTrap: FocusTrap | undefined;
 
-  /**
-   * Whether a modal is currently shown by this modal manager.
-   */
-  protected modalShown: boolean = false;
+  // Keep track of the last set focus trap
+  protected lastSetFocusTrap: number | undefined;
+
+  // Keep track if there's an modal closing
+  protected modalClosing: boolean = false;
+
+  protected keyUpListener: null | ((e: KeyboardEvent) => void) = null;
 
   view(vnode: Mithril.VnodeDOM<IModalManagerAttrs, this>): Mithril.Children {
-    const modal = this.attrs.state.modal;
-    const Tag = modal?.componentClass;
-
     return (
-      <div className="ModalManager modal fade">
-        {!!Tag && (
-          <Tag
-            key={modal?.key}
-            {...modal.attrs}
-            animateShow={this.animateShow.bind(this)}
-            animateHide={this.animateHide.bind(this)}
-            state={this.attrs.state}
+      <>
+        {this.attrs.state.modalList.map((modal, i) => {
+          const Tag = modal?.componentClass;
+
+          return (
+            <div
+              key={modal.key}
+              class="ModalManager modal"
+              data-modal-key={modal.key}
+              data-modal-number={i}
+              role="dialog"
+              aria-modal="true"
+              style={{ '--modal-number': i }}
+              aria-hidden={this.attrs.state.modal !== modal && 'true'}
+            >
+              {!!Tag && [
+                <Tag
+                  key={modal.key}
+                  {...modal.attrs}
+                  animateShow={this.animateShow.bind(this)}
+                  animateHide={this.animateHide.bind(this)}
+                  state={this.attrs.state}
+                />,
+                /* This backdrop is invisible and used for outside clicks to close the modal. */
+                <div
+                  key={modal.key}
+                  className="ModalManager-invisibleBackdrop"
+                  onclick={this.handlePossibleBackdropClick.bind(this)} />
+              ]}
+            </div>
+          );
+        })}
+
+        {this.attrs.state.backdropShown && (
+          <div
+            class="Modal-backdrop backdrop"
+            ontransitionend={this.onBackdropTransitionEnd.bind(this)}
+            data-showing={!!this.attrs.state.modalList.length}
+            style={{ '--modal-count': this.attrs.state.modalList.length }}
           />
         )}
-      </div>
+      </>
     );
   }
 
   oncreate(vnode: Mithril.VnodeDOM<IModalManagerAttrs, this>): void {
     super.oncreate(vnode);
 
-    // Ensure the modal state is notified about a closed modal, even when the
-    // DOM-based Bootstrap JavaScript code triggered the closing of the modal,
-    // e.g. via ESC key or a click on the modal backdrop.
-    this.$().on('hidden.bs.modal', this.attrs.state.close.bind(this.attrs.state));
+    this.keyUpListener = this.handleEscPress.bind(this);
+    document.body.addEventListener('keyup', this.keyUpListener);
+  }
 
-    this.focusTrap = createFocusTrap(this.element as HTMLElement, { allowOutsideClick: true });
+  onbeforeremove(vnode: Mithril.VnodeDOM<IModalManagerAttrs, this>): void {
+    super.onbeforeremove(vnode);
+
+    this.keyUpListener && document.body.removeEventListener('keyup', this.keyUpListener);
+    this.keyUpListener = null;
   }
 
   onupdate(vnode: Mithril.VnodeDOM<IModalManagerAttrs, this>): void {
@@ -57,43 +94,123 @@ export default class ModalManager extends Component<IModalManagerAttrs> {
 
     requestAnimationFrame(() => {
       try {
-        if (this.modalShown) this.focusTrap!.activate?.();
-        else this.focusTrap!.deactivate?.();
+        // Main content should gain or lose `aria-hidden` when modals are shown/removed
+        // See: http://web-accessibility.carnegiemuseums.org/code/dialogs/
+
+        if (!this.attrs.state.isModalOpen()) {
+          document.getElementById('app')?.setAttribute('aria-hidden', 'false');
+          this.focusTrap!.deactivate?.();
+          clearAllBodyScrollLocks();
+
+          return;
+        }
+
+        document.getElementById('app')?.setAttribute('aria-hidden', 'true');
+
+        // Get current dialog key
+        const dialogKey = this.attrs.state.modal!.key;
+
+        // Deactivate focus trap if there's a new dialog/closed
+        if (this.focusTrap && this.lastSetFocusTrap !== dialogKey) {
+          this.focusTrap!.deactivate?.();
+
+          clearAllBodyScrollLocks();
+        }
+
+        // Activate focus trap if there's a new dialog which is not trapped yet
+        if (this.activeDialogElement && this.lastSetFocusTrap !== dialogKey) {
+          this.focusTrap = createFocusTrap(this.activeDialogElement as HTMLElement, { allowOutsideClick: true });
+          this.focusTrap!.activate?.();
+
+          disableBodyScroll(this.activeDialogManagerElement!, { reserveScrollBarGap: true });
+        }
+
+        // Update key of current opened modal
+        this.lastSetFocusTrap = dialogKey;
       } catch {
         // We can expect errors to occur here due to the nature of mithril rendering
       }
     });
   }
 
-  animateShow(readyCallback: () => void): void {
+  /**
+   * Get current active dialog
+   */
+  private get activeDialogElement(): HTMLElement {
+    return document.body.querySelector(`.ModalManager[data-modal-key="${this.attrs.state.modal?.key}"] .Modal`) as HTMLElement;
+  }
+
+  /**
+   * Get current active dialog
+   */
+  private get activeDialogManagerElement(): HTMLElement {
+    return document.body.querySelector(`.ModalManager[data-modal-key="${this.attrs.state.modal?.key}"]`) as HTMLElement;
+  }
+
+  animateShow(readyCallback: () => void = () => {}): void {
     if (!this.attrs.state.modal) return;
 
-    const dismissible = !!this.attrs.state.modal.componentClass.isDismissible;
+    this.activeDialogElement.addEventListener(
+      'transitionend',
+      () => {
+        readyCallback();
+      },
+      { once: true }
+    );
 
-    this.modalShown = true;
-
-    // If we are opening this modal while another modal is already open,
-    // the shown event will not run, because the modal is already open.
-    // So, we need to manually trigger the readyCallback.
-    if (this.$().hasClass('in')) {
-      readyCallback();
-      return;
-    }
-
-    this.$()
-      .one('shown.bs.modal', readyCallback)
-      // @ts-expect-error: No typings available for Bootstrap modals.
-      .modal({
-        backdrop: dismissible || 'static',
-        keyboard: dismissible,
-      })
-      .modal('show');
+    requestAnimationFrame(() => {
+      this.activeDialogElement.classList.add('in');
+    });
   }
 
-  animateHide(): void {
-    // @ts-expect-error: No typings available for Bootstrap modals.
-    this.$().modal('hide');
+  animateHide(closedCallback: () => void = () => {}): void {
+    if (this.modalClosing) return;
+    this.modalClosing = true;
 
-    this.modalShown = false;
+    const afterModalClosedCallback = () => {
+      this.modalClosing = false;
+
+      // Close the dialog
+      this.attrs.state.close();
+
+      closedCallback();
+    };
+
+    this.activeDialogElement.addEventListener('transitionend', afterModalClosedCallback, { once: true });
+
+    this.activeDialogElement.classList.remove('in');
+    this.activeDialogElement.classList.add('out');
+  }
+
+  protected handleEscPress(e: KeyboardEvent): void {
+    if (!this.attrs.state.modal) return;
+
+    const dismissibleState = this.attrs.state.modal.componentClass.dismissibleOptions;
+
+    // Close the dialog if the escape key was pressed
+    // Check if closing via escape key is enabled
+    if (e.key === 'Escape' && dismissibleState.viaEscKey) {
+      e.preventDefault();
+
+      this.animateHide();
+    }
+  }
+
+  protected handlePossibleBackdropClick(e: MouseEvent): void {
+    if (!this.attrs.state.modal || !this.attrs.state.modal.componentClass.dismissibleOptions.viaBackdropClick) return;
+
+    this.animateHide();
+  }
+
+  protected onBackdropTransitionEnd(e: TransitionEvent) {
+    if (e.propertyName === 'opacity') {
+      const backdrop = e.currentTarget as HTMLDivElement;
+
+      if (backdrop.getAttribute('data-showing') === null) {
+        // Backdrop is fading out
+        this.attrs.state.backdropShown = false;
+        m.redraw();
+      }
+    }
   }
 }
diff --git a/framework/core/js/src/common/index.ts b/framework/core/js/src/common/index.ts
index 15ac4fb87..2f6f7a018 100644
--- a/framework/core/js/src/common/index.ts
+++ b/framework/core/js/src/common/index.ts
@@ -5,7 +5,6 @@ import 'expose-loader?exposes=dayjs!dayjs';
 
 import 'bootstrap/js/affix';
 import 'bootstrap/js/dropdown';
-import 'bootstrap/js/modal';
 import 'bootstrap/js/tooltip';
 import 'bootstrap/js/transition';
 import 'jquery.hotkeys/jquery.hotkeys';
diff --git a/framework/core/js/src/common/states/ModalManagerState.ts b/framework/core/js/src/common/states/ModalManagerState.ts
index a4a36093d..611e61fb9 100644
--- a/framework/core/js/src/common/states/ModalManagerState.ts
+++ b/framework/core/js/src/common/states/ModalManagerState.ts
@@ -1,5 +1,5 @@
 import type Component from '../Component';
-import Modal from '../components/Modal';
+import Modal, { IDismissibleOptions } from '../components/Modal';
 
 /**
  * Ideally, `show` would take a higher-kinded generic, ala:
@@ -8,7 +8,13 @@ import Modal from '../components/Modal';
  * https://github.com/Microsoft/TypeScript/issues/1213
  * Therefore, we have to use this ugly, messy workaround.
  */
-type UnsafeModalClass = ComponentClass<any, Modal> & { isDismissible: boolean; component: typeof Component.component };
+type UnsafeModalClass = ComponentClass<any, Modal> & { get dismissibleOptions(): IDismissibleOptions; component: typeof Component.component };
+
+type ModalItem = {
+  componentClass: UnsafeModalClass;
+  attrs?: Record<string, unknown>;
+  key: number;
+};
 
 /**
  * Class used to manage modal state.
@@ -19,11 +25,17 @@ export default class ModalManagerState {
   /**
    * @internal
    */
-  modal: null | {
-    componentClass: UnsafeModalClass;
-    attrs?: Record<string, unknown>;
-    key: number;
-  } = null;
+  modal: ModalItem | null = null;
+
+  /**
+   * @internal
+   */
+  modalList: ModalItem[] = [];
+
+  /**
+   * @internal
+   */
+  backdropShown: boolean = false;
 
   /**
    * Used to force re-initialization of modals if a modal
@@ -31,12 +43,13 @@ export default class ModalManagerState {
    */
   private key = 0;
 
-  private closeTimeout?: NodeJS.Timeout;
-
   /**
    * Shows a modal dialog.
    *
-   * If a modal is already open, the existing one will close and the new modal will replace it.
+   * If `stackModal` is `true`, the modal will be shown on top of the current modal.
+   *
+   * If a value for `stackModal` is not provided, opening a new modal will close
+   * any others currently being shown for backwards compatibility.
    *
    * @example <caption>Show a modal</caption>
    * app.modal.show(MyCoolModal, { attr: 'value' });
@@ -44,8 +57,11 @@ export default class ModalManagerState {
    * @example <caption>Show a modal from a lifecycle method (`oncreate`, `view`, etc.)</caption>
    * // This "hack" is needed due to quirks with nested redraws in Mithril.
    * setTimeout(() => app.modal.show(MyCoolModal, { attr: 'value' }), 0);
+   *
+   * @example <caption>Stacking modals</caption>
+   * app.modal.show(MyCoolStackedModal, { attr: 'value' }, true);
    */
-  show(componentClass: UnsafeModalClass, attrs: Record<string, unknown> = {}): void {
+  show(componentClass: UnsafeModalClass, attrs: Record<string, unknown> = {}, stackModal: boolean = false): void {
     if (!(componentClass.prototype instanceof Modal)) {
       // This is duplicated so that if the error is caught, an error message still shows up in the debug console.
       const invalidModalWarning = 'The ModalManager can only show Modals.';
@@ -53,28 +69,52 @@ export default class ModalManagerState {
       throw new Error(invalidModalWarning);
     }
 
-    if (this.closeTimeout) clearTimeout(this.closeTimeout);
-
-    this.modal = { componentClass, attrs, key: this.key++ };
-
+    this.backdropShown = true;
     m.redraw.sync();
+
+    // We use requestAnimationFrame here, since we need to wait for the backdrop to be added
+    // to the DOM before actually adding the modal to the modal list.
+    //
+    // This is because we use RAF inside the ModalManager onupdate lifecycle hook, and if we
+    // skip this RAF call, the hook will attempt to add a focus trap as well as lock scroll
+    // onto the newly added modal before it's in the DOM, creating an extra scrollbar.
+    requestAnimationFrame(() => {
+      // Set current modal
+      this.modal = { componentClass, attrs, key: this.key++ };
+
+      // We want to stack this modal
+      if (stackModal) {
+        // Remember previously opened modal and add new modal to the modal list
+        this.modalList.push(this.modal);
+      } else {
+        // Override last modals
+        this.modalList = [this.modal];
+      }
+
+      m.redraw();
+    });
   }
 
   /**
-   * Closes the currently open dialog, if one is open.
+   * Closes the topmost currently open dialog, if one is open.
    */
   close(): void {
     if (!this.modal) return;
 
-    // Don't hide the modal immediately, because if the consumer happens to call
-    // the `show` method straight after to show another modal dialog, it will
-    // cause Bootstrap's modal JS to misbehave. Instead we will wait for a tiny
-    // bit to give the `show` method the opportunity to prevent this from going
-    // ahead.
-    this.closeTimeout = setTimeout(() => {
+    // If there are two modals, remove the most recent one
+    if (this.modalList.length > 1) {
+      // Remove last modal from list
+      this.modalList.pop();
+
+      // Open last modal from list
+      this.modal = this.modalList[this.modalList.length - 1];
+    } else {
+      // Reset state
       this.modal = null;
-      m.redraw();
-    });
+      this.modalList = [];
+    }
+
+    m.redraw();
   }
 
   /**
diff --git a/framework/core/less/common/Modal.less b/framework/core/less/common/Modal.less
index 892057850..59a6acf3b 100644
--- a/framework/core/less/common/Modal.less
+++ b/framework/core/less/common/Modal.less
@@ -1,60 +1,58 @@
 // ------------------------------------
 // Modals
 
-// Kill the scroll on the body
-.modal-open {
-  overflow: hidden;
-}
+.Modal {
+  padding: 0;
+  border-radius: @border-radius;
 
-// Modal background
-.modal-backdrop {
-  position: fixed;
-  top: 0;
-  right: 0;
-  bottom: 0;
-  left: 0;
-  z-index: var(--zindex-modal-background);
-  background-color: var(--overlay-bg);
-  opacity: 0;
-  transition: opacity 0.2s;
+  transform: scale(0.9);
+  transition: transform 0.2s ease-out, opacity 0.2s ease-out, top 0.2s ease-out;
+  z-index: 2;
+  position: relative;
+
+  width: auto;
+  margin: 10px;
+  max-width: 600px;
 
   &.in {
+    transform: scale(1);
+  }
+}
+
+.Modal-backdrop {
+  background: var(--overlay-bg);
+  position: fixed;
+  top: 0;
+  left: 0;
+  right: 0;
+  bottom: 0;
+  opacity: 0;
+  transition: opacity 0.2s ease-out;
+  z-index: ~"calc(var(--zindex-modal) + var(--modal-count) - 2)";
+
+  &[data-showing] {
     opacity: 1;
   }
 }
 
-// Container that the modal scrolls within
 .ModalManager {
-  display: none;
-  overflow: hidden;
   position: fixed;
   top: 0;
   right: 0;
   bottom: 0;
   left: 0;
-  z-index: var(--zindex-modal);
+  z-index: ~"calc(var(--zindex-modal) + var(--modal-number))";
   -webkit-overflow-scrolling: touch;
-
-  // When fading in the modal, animate it to slide down
-  .Modal {
-    transform: scale(0.9);
-    transition: transform 0.2s ease-out;
-  }
-  &.in .Modal {
-    transform: scale(1);
-  }
-}
-.modal-open .ModalManager {
   overflow-x: hidden;
   overflow-y: auto;
-}
 
-// Shell div to position the modal with bottom padding
-.Modal {
-  position: relative;
-  width: auto;
-  margin: 10px;
-  max-width: 600px;
+  &-invisibleBackdrop {
+    position: absolute;
+    top: 0;
+    right: 0;
+    bottom: 0;
+    left: 0;
+  }
 }
 
 // Actual modal
@@ -129,9 +127,6 @@
 }
 
 @media @phone {
-  .ModalManager.fade {
-    opacity: 1;
-  }
   .ModalManager {
     position: fixed;
     left: 0;
@@ -139,24 +134,28 @@
     bottom: 0;
     top: 0;
     overflow: auto;
-    transition: transform 0.2s ease-out;
-    transform: translate(0, 100vh);
 
-    &.in {
-      -webkit-transform: none !important;
-              transform: none !important;
-    }
     &:before {
       content: " ";
       .header-background();
       position: absolute;
+      z-index: 2;
     }
   }
   .Modal {
     max-width: 100%;
     margin: 0;
     -webkit-transform: none !important;
-            transform: none !important;
+    transform: none !important;
+    top: 100vh;
+
+    &.fade {
+      opacity: 1;
+    }
+
+    &.in {
+      top: 0;
+    }
   }
   .Modal-content {
     border-radius: 0;
@@ -174,19 +173,20 @@
 
 @media @tablet-up {
   .Modal {
+    border-radius: var(--border-radius);
+    box-shadow: 0 7px 15px var(--shadow-color);
+    width: 100%;
+    max-width: 600px;
     margin: 120px auto;
   }
   .Modal-close {
     position: absolute;
     right: 10px;
     top: 10px;
-    z-index: 1;
+    z-index: 2;
   }
   .Modal-content {
-
-    border: 0;
     border-radius: var(--border-radius);
-    box-shadow: 0 7px 15px var(--shadow-color);
   }
   .Modal--small {
     max-width: 375px;
diff --git a/yarn.lock b/yarn.lock
index cb1c63b3c..5c71bbae8 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -1112,6 +1112,11 @@
     prop-types "^15.7.2"
     react-is "^16.6.3"
 
+"@types/body-scroll-lock@^3.1.0":
+  version "3.1.0"
+  resolved "https://registry.yarnpkg.com/@types/body-scroll-lock/-/body-scroll-lock-3.1.0.tgz#435f6abf682bf58640e1c2ee5978320b891970e7"
+  integrity sha512-3owAC4iJub5WPqRhxd8INarF2bWeQq1yQHBgYhN0XLBJMpd5ED10RrJ3aKiAwlTyL5wK7RkBD4SZUQz2AAAMdA==
+
 "@types/eslint-scope@^3.7.3":
   version "3.7.3"
   resolved "https://registry.yarnpkg.com/@types/eslint-scope/-/eslint-scope-3.7.3.tgz#125b88504b61e3c8bc6f870882003253005c3224"
@@ -1488,6 +1493,11 @@ big.js@^5.2.2:
   resolved "https://registry.yarnpkg.com/big.js/-/big.js-5.2.2.tgz#65f0af382f578bcdc742bd9c281e9cb2d7768328"
   integrity sha512-vyL2OymJxmarO8gxMr0mhChsO9QGwhynfuu4+MHTAW6czfq9humCB7rKpUjDd9YUiDPU4mzpyupFSvOClAwbmQ==
 
+body-scroll-lock@^4.0.0-beta.0:
+  version "4.0.0-beta.0"
+  resolved "https://registry.yarnpkg.com/body-scroll-lock/-/body-scroll-lock-4.0.0-beta.0.tgz#4f78789d10e6388115c0460cd6d7d4dd2bbc4f7e"
+  integrity sha512-a7tP5+0Mw3YlUJcGAKUqIBkYYGlYxk2fnCasq/FUph1hadxlTRjF+gAcZksxANnaMnALjxEddmSi/H3OR8ugcQ==
+
 bootstrap@^3.4.1:
   version "3.4.1"
   resolved "https://registry.yarnpkg.com/bootstrap/-/bootstrap-3.4.1.tgz#c3a347d419e289ad11f4033e3c4132b87c081d72"