UX: Use Visual Viewport API for iOS composer height

This applies to iPhones running iOS 13+.
Previous technique remains in place for iOS 12 and below.

Note that this does not apply to iPads on iOS 13 due to Apple no longer
identifying iPads in the user agent string.

app/assets/javascripts/discourse/components/composer-body.js.es6

@@ -131,6 +131,25 @@ export default Ember.Component.extend(KeyEnterEscape, {
       $document.on(DRAG_EVENTS, throttledPerformDrag);
       $document.on(END_EVENTS, endDrag);
     });
+
+    if (window.visualViewport !== undefined) {
+      this.viewportResize();
+      window.visualViewport.addEventListener("resize", this.viewportResize);
+    }
+  },
+
+  viewportResize() {
+    const composerVH = window.visualViewport.height * 0.01;
+
+    if (window.visualViewport.height !== window.innerHeight) {
+      document.documentElement.classList.add("keyboard-visible");
+    } else {
+      document.documentElement.classList.remove("keyboard-visible");
+    }
+    document.documentElement.style.setProperty(
+      "--composer-vh",
+      `${composerVH}px`
+    );
   },
 
   didInsertElement() {
@@ -155,6 +174,9 @@ export default Ember.Component.extend(KeyEnterEscape, {
   willDestroyElement() {
     this._super(...arguments);
     this.appEvents.off("composer:resize", this, this.resize);
+    if (window.visualViewport !== undefined) {
+      window.visualViewport.removeEventListener("resize", this.viewportResize);
+    }
   },
 
   click() {

app/assets/javascripts/discourse/lib/safari-hacks.js.es6

@@ -1,6 +1,9 @@
 import debounce from "discourse/lib/debounce";
 import { isAppleDevice, safariHacksDisabled } from "discourse/lib/utilities";
 
+// TODO: remove calcHeight once iOS 13 adoption > 90%
+// In iOS 13 and up we use visualViewport API to calculate height
+
 // we can't tell what the actual visible window height is
 // because we cannot account for the height of the mobile keyboard
 // and any other mobile autocomplete UI that may appear
@@ -89,9 +92,14 @@ function positioningWorkaround($fixedElement) {
 
       fixedElement.style.position = "";
       fixedElement.style.top = "";
-      fixedElement.style.height = oldHeight;
 
-      Ember.run.later(() => $(fixedElement).removeClass("no-transition"), 500);
+      if (window.visualViewport === undefined) {
+        fixedElement.style.height = oldHeight;
+        Ember.run.later(
+          () => $(fixedElement).removeClass("no-transition"),
+          500
+        );
+      }
 
       $(window).scrollTop(originalScrollTop);
 
@@ -165,10 +173,11 @@ function positioningWorkaround($fixedElement) {
 
     fixedElement.style.top = "0px";
 
-    const height = calcHeight();
-    fixedElement.style.height = height + "px";
-
-    $(fixedElement).addClass("no-transition");
+    if (window.visualViewport === undefined) {
+      const height = calcHeight();
+      fixedElement.style.height = height + "px";
+      $(fixedElement).addClass("no-transition");
+    }
 
     evt.preventDefault();
     this.focus();

app/assets/stylesheets/mobile/compose.scss

@@ -21,6 +21,14 @@
     height: 250px;
     &.edit-title {
       height: 100%;
+      height: calc(var(--composer-vh, 1vh) * 100);
+    }
+  }
+
+  html.keyboard-visible &.open {
+    height: calc(var(--composer-vh, 1vh) * 100);
+    .reply-area {
+      padding-bottom: 0px;
     }
   }