Merge pull request #4931 from matrix-org/travis/room-list/sticky-headers

Improve performance and stability in sticky headers for new room list

res/css/structures/_LeftPanel2.scss

@@ -122,16 +122,19 @@ $tagPanelWidth: 70px; // only applies in this file, used for calculations
         }
 
         .mx_LeftPanel2_roomListWrapper {
+            // Create a flexbox to ensure the containing items cause appropriate overflow.
             display: flex;
+
             flex-grow: 1;
             overflow: hidden;
             min-height: 0;
+            margin-top: 12px; // so we're not up against the search/filter
 
-            &.stickyBottom {
+            &.mx_LeftPanel2_roomListWrapper_stickyBottom {
                 padding-bottom: 32px;
             }
 
-            &.stickyTop {
+            &.mx_LeftPanel2_roomListWrapper_stickyTop {
                 padding-top: 32px;
             }
         }

res/css/views/rooms/_RoomSublist2.scss

@@ -24,10 +24,6 @@ limitations under the License.
     margin-left: 8px;
     width: 100%;
 
-    &:first-child {
-        margin-top: 12px; // so we're not up against the search/filter
-    }
-
     .mx_RoomSublist2_headerContainer {
         // Create a flexbox to make alignment easy
         display: flex;
@@ -49,10 +45,15 @@ limitations under the License.
         padding-bottom: 8px;
         height: 24px;
 
+        // Hide the header container if the contained element is stickied.
+        // We don't use display:none as that causes the header to go away too.
+        &.mx_RoomSublist2_headerContainer_hasSticky {
+            height: 0;
+        }
+
         .mx_RoomSublist2_stickable {
             flex: 1;
             max-width: 100%;
-            z-index: 2; // Prioritize headers in the visible list over sticky ones
 
             // Create a flexbox to make ordering easy
             display: flex;
@@ -64,7 +65,6 @@ limitations under the License.
             // when sticky scrolls instead of collapses the list.
             &.mx_RoomSublist2_headerContainer_sticky {
                 position: fixed;
-                z-index: 1; // over top of other elements, but still under the ones in the visible list
                 height: 32px; // to match the header container
                 // width set by JS
             }

src/components/structures/LeftPanel2.tsx

@@ -115,86 +115,130 @@ export default class LeftPanel2 extends React.Component<IProps, IState> {
     };
 
     private handleStickyHeaders(list: HTMLDivElement) {
-        // TODO: Evaluate if this has any performance benefit or detriment.
-        // See https://github.com/vector-im/riot-web/issues/14035
-
         if (this.isDoingStickyHeaders) return;
         this.isDoingStickyHeaders = true;
-        if (window.requestAnimationFrame) {
         window.requestAnimationFrame(() => {
             this.doStickyHeaders(list);
             this.isDoingStickyHeaders = false;
         });
-        } else {
-            this.doStickyHeaders(list);
-            this.isDoingStickyHeaders = false;
-        }
     }
 
     private doStickyHeaders(list: HTMLDivElement) {
-        const rlRect = list.getBoundingClientRect();
+        const topEdge = list.scrollTop;
-        const bottom = rlRect.bottom;
+        const bottomEdge = list.offsetHeight + list.scrollTop;
-        const top = rlRect.top;
         const sublists = list.querySelectorAll<HTMLDivElement>(".mx_RoomSublist2");
-        const headerRightMargin = 24; // calculated from margins and widths to align with non-sticky tiles
 
-        const headerStickyWidth = rlRect.width - headerRightMargin;
+        const headerRightMargin = 16; // calculated from margins and widths to align with non-sticky tiles
+        const headerStickyWidth = list.clientWidth - headerRightMargin;
+
+        // We track which styles we want on a target before making the changes to avoid
+        // excessive layout updates.
+        const targetStyles = new Map<HTMLDivElement, {
+            stickyTop?: boolean;
+            stickyBottom?: boolean;
+            makeInvisible?: boolean;
+        }>();
 
-        let gotBottom = false;
         let lastTopHeader;
+        let firstBottomHeader;
         for (const sublist of sublists) {
-            const slRect = sublist.getBoundingClientRect();
-
             const header = sublist.querySelector<HTMLDivElement>(".mx_RoomSublist2_stickable");
             header.style.removeProperty("display"); // always clear display:none first
 
-            if (slRect.top + HEADER_HEIGHT > bottom && !gotBottom) {
+            // When an element is <=40% off screen, make it take over
-                header.classList.add("mx_RoomSublist2_headerContainer_sticky");
+            const offScreenFactor = 0.4;
-                header.classList.add("mx_RoomSublist2_headerContainer_stickyBottom");
+            const isOffTop = (sublist.offsetTop + (offScreenFactor * HEADER_HEIGHT)) <= topEdge;
-                header.style.width = `${headerStickyWidth}px`;
+            const isOffBottom = (sublist.offsetTop + (offScreenFactor * HEADER_HEIGHT)) >= bottomEdge;
-                header.style.removeProperty("top");
+
-                gotBottom = true;
+            if (isOffTop || sublist === sublists[0]) {
-            } else if (((slRect.top - (HEADER_HEIGHT * 0.6) + HEADER_HEIGHT) < top) || sublist === sublists[0]) {
+                targetStyles.set(header, { stickyTop: true });
-                // the header should become sticky once it is 60% or less out of view at the top.
-                // We also add HEADER_HEIGHT because the sticky header is put above the scrollable area,
-                // into the padding of .mx_LeftPanel2_roomListWrapper,
-                // by subtracting HEADER_HEIGHT from the top below.
-                // We also always try to make the first sublist header sticky.
-                header.classList.add("mx_RoomSublist2_headerContainer_sticky");
-                header.classList.add("mx_RoomSublist2_headerContainer_stickyTop");
-                header.style.width = `${headerStickyWidth}px`;
-                header.style.top = `${rlRect.top - HEADER_HEIGHT}px`;
                 if (lastTopHeader) {
                     lastTopHeader.style.display = "none";
+                    targetStyles.set(lastTopHeader, { makeInvisible: true });
                 }
                 lastTopHeader = header;
+            } else if (isOffBottom && !firstBottomHeader) {
+                targetStyles.set(header, { stickyBottom: true });
+                firstBottomHeader = header;
             } else {
+                targetStyles.set(header, {}); // nothing == clear
+            }
+        }
+
+        // Run over the style changes and make them reality. We check to see if we're about to
+        // cause a no-op update, as adding/removing properties that are/aren't there cause
+        // layout updates.
+        for (const header of targetStyles.keys()) {
+            const style = targetStyles.get(header);
+            const headerContainer = header.parentElement; // .mx_RoomSublist2_headerContainer
+
+            if (style.makeInvisible) {
+                // we will have already removed the 'display: none', so add it back.
+                header.style.display = "none";
+                continue; // nothing else to do, even if sticky somehow
+            }
+
+            if (style.stickyTop) {
+                if (!header.classList.contains("mx_RoomSublist2_headerContainer_stickyTop")) {
+                    header.classList.add("mx_RoomSublist2_headerContainer_stickyTop");
+                }
+
+                const newTop = `${list.parentElement.offsetTop}px`;
+                if (header.style.top !== newTop) {
+                    header.style.top = newTop;
+                }
+            } else if (style.stickyBottom) {
+                if (!header.classList.contains("mx_RoomSublist2_headerContainer_stickyBottom")) {
+                    header.classList.add("mx_RoomSublist2_headerContainer_stickyBottom");
+                }
+            }
+
+            if (style.stickyTop || style.stickyBottom) {
+                if (!header.classList.contains("mx_RoomSublist2_headerContainer_sticky")) {
+                    header.classList.add("mx_RoomSublist2_headerContainer_sticky");
+                }
+                if (!headerContainer.classList.contains("mx_RoomSublist2_headerContainer_hasSticky")) {
+                    headerContainer.classList.add("mx_RoomSublist2_headerContainer_hasSticky");
+                }
+
+                const newWidth = `${headerStickyWidth}px`;
+                if (header.style.width !== newWidth) {
+                    header.style.width = newWidth;
+                }
+            } else if (!style.stickyTop && !style.stickyBottom) {
+                if (header.classList.contains("mx_RoomSublist2_headerContainer_sticky")) {
                     header.classList.remove("mx_RoomSublist2_headerContainer_sticky");
+                }
+                if (header.classList.contains("mx_RoomSublist2_headerContainer_stickyTop")) {
                     header.classList.remove("mx_RoomSublist2_headerContainer_stickyTop");
+                }
+                if (header.classList.contains("mx_RoomSublist2_headerContainer_stickyBottom")) {
                     header.classList.remove("mx_RoomSublist2_headerContainer_stickyBottom");
-                header.style.removeProperty("width");
+                }
-                header.style.removeProperty("top");
+                if (headerContainer.classList.contains("mx_RoomSublist2_headerContainer_hasSticky")) {
+                    headerContainer.classList.remove("mx_RoomSublist2_headerContainer_hasSticky");
+                }
+                if (header.style.width) {
+                    header.style.removeProperty('width');
+                }
+                if (header.style.top) {
+                    header.style.removeProperty('top');
+                }
             }
         }
 
         // add appropriate sticky classes to wrapper so it has
         // the necessary top/bottom padding to put the sticky header in
-        const listWrapper = list.parentElement;
+        const listWrapper = list.parentElement; // .mx_LeftPanel2_roomListWrapper
-        if (gotBottom) {
-            listWrapper.classList.add("stickyBottom");
-        } else {
-            listWrapper.classList.remove("stickyBottom");
-        }
         if (lastTopHeader) {
-            listWrapper.classList.add("stickyTop");
+            listWrapper.classList.add("mx_LeftPanel2_roomListWrapper_stickyTop");
         } else {
-            listWrapper.classList.remove("stickyTop");
+            listWrapper.classList.remove("mx_LeftPanel2_roomListWrapper_stickyTop");
         }
-
+        if (firstBottomHeader) {
-        // ensure scroll doesn't go above the gap left by the header of
+            listWrapper.classList.add("mx_LeftPanel2_roomListWrapper_stickyBottom");
-        // the first sublist always being sticky if no other header is sticky
+        } else {
-        if (list.scrollTop < HEADER_HEIGHT) {
+            listWrapper.classList.remove("mx_LeftPanel2_roomListWrapper_stickyBottom");
-            list.scrollTop = HEADER_HEIGHT;
         }
     }