Merge pull request #4974 from matrix-org/jryans/sticky-pop

Tweak sticky header hiding to avoid pop

res/css/views/rooms/_RoomSublist2.scss

@@ -47,12 +47,6 @@ 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%;
@@ -180,6 +174,15 @@ limitations under the License.
         }
     }
 
+    // In the general case, we leave height of headers alone even if sticky, so
+    // that the sublists below them do not jump. However, that leaves a gap
+    // when scrolled to the top above the first sublist (whose header can only
+    // ever stick to top), so we force height to 0 for only that first header.
+    // See also https://github.com/vector-im/riot-web/issues/14429.
+    &:first-child .mx_RoomSublist2_headerContainer {
+        height: 0;
+    }
+
     .mx_RoomSublist2_resizeBox {
         position: relative;
 

src/components/structures/LeftPanel2.tsx

@@ -165,7 +165,6 @@ export default class LeftPanel2 extends React.Component<IProps, IState> {
         // 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.
@@ -205,9 +204,6 @@ export default class LeftPanel2 extends React.Component<IProps, IState> {
                 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) {
@@ -217,9 +213,6 @@ export default class LeftPanel2 extends React.Component<IProps, IState> {
                 if (header.classList.contains("mx_RoomSublist2_headerContainer_sticky")) {
                     header.classList.remove("mx_RoomSublist2_headerContainer_sticky");
                 }
-                if (headerContainer.classList.contains("mx_RoomSublist2_headerContainer_hasSticky")) {
-                    headerContainer.classList.remove("mx_RoomSublist2_headerContainer_hasSticky");
-                }
                 if (header.style.width) {
                     header.style.removeProperty('width');
                 }