PERF: Refactor slide-in menu sizing for improved performance (#20377)

Using Javascript to read and recalculate sizing is prone to causing 'forced reflows', which are very expensive, especially on slower devices. This PR refactors the slide-in menus so that all of the height calculation is done using CSS. This is made possible by the new dvh (dynamic view height) units and env(safe-area-inset-bottom), both of which are supported on all of our target browsers.

In tests on a moto g50, on a sidebar with 16 categories, 15 tags, and 2 chat channels, this improves the sidebar opening time by around 50ms (6%).

app/assets/javascripts/discourse/app/components/site-header.js

@@ -54,6 +54,12 @@ const SiteHeaderComponent = MountWidget.extend(
     },
 
     _animateOpening(panel) {
+      window.requestAnimationFrame(
+        this._setAnimateOpeningProperties.bind(this, panel)
+      );
+    },
+
+    _setAnimateOpeningProperties(panel) {
       const headerCloak = document.querySelector(".header-cloak");
       panel.classList.add("animate");
       headerCloak.classList.add("animate");
@@ -67,13 +73,16 @@ const SiteHeaderComponent = MountWidget.extend(
     },
 
     _animateClosing(panel, menuOrigin) {
-      const windowWidth = document.body.offsetWidth;
       this._animate = true;
       const headerCloak = document.querySelector(".header-cloak");
       panel.classList.add("animate");
       headerCloak.classList.add("animate");
-      const offsetDirection = menuOrigin === "left" ? -1 : 1;
+      if (menuOrigin === "left") {
-      panel.style.setProperty("--offset", `${offsetDirection * windowWidth}px`);
+        panel.style.setProperty("--offset", `-100vw`);
+      } else {
+        panel.style.setProperty("--offset", `100vw`);
+      }
+
       headerCloak.style.setProperty("--opacity", 0);
       this._scheduledRemoveAnimate = discourseLater(() => {
         panel.classList.remove("animate");
@@ -365,7 +374,6 @@ const SiteHeaderComponent = MountWidget.extend(
         return;
       }
 
-      const windowWidth = document.body.offsetWidth;
       const viewMode =
         this.site.mobileView || this.site.narrowDesktopView
           ? "slide-in"
@@ -374,9 +382,6 @@ const SiteHeaderComponent = MountWidget.extend(
       menuPanels.forEach((panel) => {
         const headerCloak = document.querySelector(".header-cloak");
         let width = parseInt(panel.getAttribute("data-max-width"), 10) || 300;
-        if (windowWidth - width < 50) {
-          width = windowWidth - 50;
-        }
         if (this._panMenuOffset) {
           this._panMenuOffset = -width;
         }
@@ -384,77 +389,23 @@ const SiteHeaderComponent = MountWidget.extend(
         panel.classList.remove("drop-down");
         panel.classList.remove("slide-in");
         panel.classList.add(viewMode);
+
         if (this._animate || this._panMenuOffset !== 0) {
           if (
             (this.site.mobileView || this.site.narrowDesktopView) &&
             panel.parentElement.classList.contains(this._leftMenuClass())
           ) {
             this._panMenuOrigin = "left";
-            panel.style.setProperty("--offset", `${-windowWidth}px`);
+            panel.style.setProperty("--offset", `-100vw`);
           } else {
             this._panMenuOrigin = "right";
-            panel.style.setProperty("--offset", `${windowWidth}px`);
+            panel.style.setProperty("--offset", `100vw`);
           }
           headerCloak.style.setProperty("--opacity", 0);
         }
 
-        const panelBody = panel.querySelector(".panel-body");
+        if (viewMode === "slide-in") {
-
-        // We use a mutationObserver to check for style changes, so it's important
-        // we don't set it if it doesn't change. Same goes for the panelBody!
-
-        if (!this.site.mobileView && !this.site.narrowDesktopView) {
-          const buttonPanel = document.querySelectorAll("header ul.icons");
-          if (buttonPanel.length === 0) {
-            return;
-          }
-
-          // These values need to be set here, not in the css file - this is to deal with the
-          // possibility of the window being resized and the menu changing from .slide-in to .drop-down.
-          if (panel.style.top !== "100%" || panel.style.height !== "auto") {
-            panel.style.setProperty("top", "100%");
-            panel.style.setProperty("height", "auto");
-          }
-        } else {
           headerCloak.style.display = "block";
-
-          const menuTop = headerTop();
-
-          const winHeightOffset = this.currentUser?.redesigned_user_menu_enabled
-            ? 0
-            : 16;
-          let initialWinHeight = window.innerHeight;
-          const winHeight = initialWinHeight - winHeightOffset;
-
-          let height = winHeight - menuTop;
-
-          const isIPadApp = document.body.classList.contains("footer-nav-ipad"),
-            heightProp = isIPadApp ? "max-height" : "height",
-            iPadOffset = 10;
-
-          if (isIPadApp) {
-            height = winHeight - menuTop - iPadOffset;
-          }
-
-          if (panelBody.style.height !== "100%") {
-            panelBody.style.setProperty("height", "100%");
-          }
-          if (
-            panel.style.top !== `${menuTop}px` ||
-            panel.style[heightProp] !== `${height}px`
-          ) {
-            panel.style.top = `${menuTop}px`;
-            panel.style.setProperty(heightProp, `${height}px`);
-            if (headerCloak) {
-              headerCloak.style.top = `${menuTop}px`;
-            }
-          }
-        }
-
-        // TODO: remove the if condition when redesigned_user_menu_enabled is
-        // removed
-        if (!panel.classList.contains("revamped")) {
-          panel.style.setProperty("width", `${width}px`);
         }
         if (this._animate) {
           this._animateOpening(panel);
@@ -488,12 +439,20 @@ export default SiteHeaderComponent.extend({
 
     this.appEvents.on("site-header:force-refresh", this, "queueRerender");
 
-    const header = document.querySelector(".d-header-wrap");
+    const headerWrap = document.querySelector(".d-header-wrap");
-    if (header) {
+    let header;
+    if (headerWrap) {
       schedule("afterRender", () => {
+        header = headerWrap.querySelector("header.d-header");
+        const headerOffset = headerWrap.offsetHeight;
+        const headerTop = header.offsetTop;
         document.documentElement.style.setProperty(
           "--header-offset",
-          `${header.offsetHeight}px`
+          `${headerOffset}px`
+        );
+        document.documentElement.style.setProperty(
+          "--header-top",
+          `${headerTop}px`
         );
       });
     }
@@ -502,15 +461,21 @@ export default SiteHeaderComponent.extend({
       this._resizeObserver = new ResizeObserver((entries) => {
         for (let entry of entries) {
           if (entry.contentRect) {
+            const headerOffset = entry.contentRect.height;
+            const headerTop = header.offsetTop;
             document.documentElement.style.setProperty(
               "--header-offset",
-              entry.contentRect.height + "px"
+              `${headerOffset}px`
+            );
+            document.documentElement.style.setProperty(
+              "--header-top",
+              `${headerTop}px`
             );
           }
         }
       });
 
-      this._resizeObserver.observe(header);
+      this._resizeObserver.observe(headerWrap);
     }
   },
 
@@ -521,8 +486,3 @@ export default SiteHeaderComponent.extend({
     this.appEvents.off("site-header:force-refresh", this, "queueRerender");
   },
 });
-
-export function headerTop() {
-  const header = document.querySelector("header.d-header");
-  return header.offsetTop ? header.offsetTop : 0;
-}

app/assets/javascripts/discourse/tests/acceptance/mobile-pan-test.js

@@ -7,6 +7,10 @@ import {
 import { click, triggerEvent, visit } from "@ember/test-helpers";
 
 async function triggerSwipeStart(touchTarget) {
+  const emberTesting = document.querySelector("#ember-testing-container");
+  emberTesting.scrollTop = 0;
+  emberTesting.scrollLeft = 0;
+
   // Other tests are shown in a transformed viewport, and this is a multiple for the offsets
   let scale = parseFloat(
     window

app/assets/stylesheets/common/base/menu-panel.scss

@@ -29,6 +29,8 @@
   overflow: hidden;
   display: flex;
   flex-direction: column;
+  box-sizing: border-box;
+
   hr {
     margin: 3px 0;
   }
@@ -689,7 +691,7 @@ body.footer-nav-ipad {
   background-color: black;
   --opacity: 0.5;
   opacity: var(--opacity);
-  top: 0;
+  top: var(--header-top);
   left: 0;
   display: none;
   touch-action: pan-y pinch-zoom;
@@ -702,6 +704,23 @@ body.footer-nav-ipad {
 }
 
 .menu-panel.slide-in {
+  top: var(--header-top);
+  box-sizing: border-box;
+
+  /* Use dvh where supported, with fallback to vh */
+  --100dvh: 100vh;
+  --100dvh: 100dvh;
+
+  --base-height: calc(
+    var(--100dvh) - var(--header-top) - env(safe-area-inset-bottom, 0px)
+  );
+
+  height: var(--base-height);
+
+  body.footer-nav-ipad & {
+    height: calc(var(--base-height) - var(--footer-nav-height));
+  }
+
   transform: translateX(var(--offset));
   @media (prefers-reduced-motion: no-preference) {
     &.animate {