Support horizontal scrolling on breadcrumbs

Fixes element-hq/element-web#8714
Fixes element-hq/element-web#8890
Fixes element-hq/element-web#9034
Fixes element-hq/element-web#8954

This turned out to be much more complicated than it needed to be. We use an IndicatorScrollbar to do all the math for us and some minor changes have been made so it can flag left/right overflow. The complicated part is the css changes which make the gradients work: unlike the RoomSubList, we have to calculate the offset of the indicators (gradients) on our own because position:sticky doesn't work horizontally.

The changes to the css (well, mostly pointer-events:none) make it so the gradient doesn't interfere with the room avatars. 

9034 and 8954 are fixed by this because they represent an overflow-x:none style breakage where browsers won't let you scroll without a scrollbar. The gradient offset problem is also demonstrated in 8954.

res/css/views/rooms/_RoomBreadcrumbs.scss

@@ -15,33 +15,29 @@ limitations under the License.
 */
 
 .mx_RoomBreadcrumbs {
-    overflow-x: auto;
     position: relative;
-    height: 32px;
+    height: 42px;
     margin: 8px;
     margin-bottom: 0;
-
-    overflow-x: hidden;
+    overflow-x: visible;
     display: flex;
     flex-direction: row;
 
-    > * {
-        margin-left: 4px;
+    .mx_AutoHideScrollbar_offset {
+        display: flex;
+        flex-direction: row;
+        height: 100%;
     }
 
-    &::after {
-        content: "";
-        position: absolute;
-        width: 15px;
-        top: 0;
-        right: 0;
-        height: 100%;
-        background: linear-gradient(to right, $panel-gradient);
+    .mx_RoomBreadcrumbs_crumb {
+        margin-left: 4px;
+        height: 32px;
+        display: inline-block;
+        transition: transform 0.3s, width 0.3s;
     }
 
     .mx_RoomBreadcrumbs_animate {
         margin-left: 0;
-        transition: transform 0.3s, width 0.3s;
         width: 32px;
         transform: scale(1);
     }
@@ -50,5 +46,34 @@ limitations under the License.
         width: 0;
         transform: scale(0);
     }
+
+
+    // Note: we have to manually control the gradient and stuff, but the IndicatorScrollbar
+    // will deal with left/right positioning for us. Normally we'd use position:sticky on
+    // a few key elements, however that doesn't work in horizontal scrolling scenarios.
+
+    .mx_IndicatorScrollbar_leftOverflowIndicator,
+    .mx_IndicatorScrollbar_rightOverflowIndicator {
+        display: none;
+    }
+
+    &.mx_IndicatorScrollbar_leftOverflow .mx_IndicatorScrollbar_leftOverflowIndicator,
+    &.mx_IndicatorScrollbar_rightOverflow .mx_IndicatorScrollbar_rightOverflowIndicator {
+        position: absolute;
+        top: 0;
+        bottom: 0;
+        width: 15px;
+        display: block;
+        pointer-events: none;
+        z-index: 100;
+    }
+
+    .mx_IndicatorScrollbar_leftOverflowIndicator {
+        background: linear-gradient(to left, $panel-gradient);
+    }
+
+    .mx_IndicatorScrollbar_rightOverflowIndicator {
+        background: linear-gradient(to right, $panel-gradient);
+    }
 }
 

src/components/structures/IndicatorScrollbar.js

@@ -15,16 +15,29 @@ limitations under the License.
 */
 
 import React from "react";
+import PropTypes from "prop-types";
 import AutoHideScrollbar from "./AutoHideScrollbar";
 
 export default class IndicatorScrollbar extends React.Component {
+    static PropTypes = {
+        // If true, the scrollbar will append mx_IndicatorScrollbar_leftOverflowIndicator
+        // and mx_IndicatorScrollbar_rightOverflowIndicator elements to the list for positioning
+        // by the parent element.
+        trackHorizontalOverflow: PropTypes.bool,
+    };
+
     constructor(props) {
         super(props);
         this._collectScroller = this._collectScroller.bind(this);
         this._collectScrollerComponent = this._collectScrollerComponent.bind(this);
         this.checkOverflow = this.checkOverflow.bind(this);
         this._scrollElement = null;
         this._autoHideScrollbar = null;
+
+        this.state = {
+            leftIndicatorOffset: 0,
+            rightIndicatorOffset: 0,
+        };
     }
 
     _collectScroller(scroller) {
@@ -43,6 +56,10 @@ export default class IndicatorScrollbar extends React.Component {
         const hasTopOverflow = this._scrollElement.scrollTop > 0;
         const hasBottomOverflow = this._scrollElement.scrollHeight >
             (this._scrollElement.scrollTop + this._scrollElement.clientHeight);
+        const hasLeftOverflow = this._scrollElement.scrollLeft > 0;
+        const hasRightOverflow = this._scrollElement.scrollWidth >
+            (this._scrollElement.scrollLeft + this._scrollElement.clientWidth);
+
         if (hasTopOverflow) {
             this._scrollElement.classList.add("mx_IndicatorScrollbar_topOverflow");
         } else {
@@ -53,10 +70,30 @@ export default class IndicatorScrollbar extends React.Component {
         } else {
             this._scrollElement.classList.remove("mx_IndicatorScrollbar_bottomOverflow");
         }
+        if (hasLeftOverflow) {
+            this._scrollElement.classList.add("mx_IndicatorScrollbar_leftOverflow");
+        } else {
+            this._scrollElement.classList.remove("mx_IndicatorScrollbar_leftOverflow");
+        }
+        if (hasRightOverflow) {
+            this._scrollElement.classList.add("mx_IndicatorScrollbar_rightOverflow");
+        } else {
+            this._scrollElement.classList.remove("mx_IndicatorScrollbar_rightOverflow");
+        }
 
         if (this._autoHideScrollbar) {
             this._autoHideScrollbar.checkOverflow();
         }
+
+        if (this.props.trackHorizontalOverflow) {
+            this.setState({
+                // Offset from absolute position of the container
+                leftIndicatorOffset: hasLeftOverflow ? `${this._scrollElement.scrollLeft}px` : 0,
+
+                // Negative because we're coming from the right
+                rightIndicatorOffset: hasRightOverflow ? `-${this._scrollElement.scrollLeft}px` : 0,
+            });
+        }
     }
 
     getScrollTop() {
@@ -70,12 +107,21 @@ export default class IndicatorScrollbar extends React.Component {
     }
 
     render() {
+        const leftIndicatorStyle = {left: this.state.leftIndicatorOffset};
+        const rightIndicatorStyle = {right: this.state.rightIndicatorOffset};
+        const leftOverflowIndicator = this.props.trackHorizontalOverflow
+            ? <div className="mx_IndicatorScrollbar_leftOverflowIndicator" style={leftIndicatorStyle} /> : null;
+        const rightOverflowIndicator = this.props.trackHorizontalOverflow
+            ? <div className="mx_IndicatorScrollbar_rightOverflowIndicator" style={rightIndicatorStyle} /> : null;
+
         return (<AutoHideScrollbar
             ref={this._collectScrollerComponent}
             wrappedRef={this._collectScroller}
             {... this.props}
         >
+            { leftOverflowIndicator }
             { this.props.children }
+            { rightOverflowIndicator }
         </AutoHideScrollbar>);
     }
 }

src/components/views/rooms/RoomBreadcrumbs.js

@@ -102,6 +102,8 @@ export default class RoomBreadcrumbs extends React.Component {
 
     render() {
         const Tooltip = sdk.getComponent('elements.Tooltip');
+        const IndicatorScrollbar = sdk.getComponent('structures.IndicatorScrollbar');
+
         // check for collapsed here and
         // not at parent so we keep
         // rooms in our state
@@ -113,6 +115,7 @@ export default class RoomBreadcrumbs extends React.Component {
         const avatars = rooms.map(({room, animated, hover}, i) => {
             const isFirst = i === 0;
             const classes = classNames({
+                "mx_RoomBreadcrumbs_crumb": true,
                 "mx_RoomBreadcrumbs_preAnimate": isFirst && !animated,
                 "mx_RoomBreadcrumbs_animate": isFirst,
             });
@@ -130,6 +133,11 @@ export default class RoomBreadcrumbs extends React.Component {
                 </AccessibleButton>
             );
         });
-        return (<div className="mx_RoomBreadcrumbs">{ avatars }</div>);
+        return (
+            <IndicatorScrollbar ref="scroller" className="mx_RoomBreadcrumbs" onScroll={ this._onScroll }
+                                trackHorizontalOverflow={true}>
+                { avatars }
+            </IndicatorScrollbar>
+        );
     }
 }