add explore button next to filter field

res/css/structures/_LeftPanel.scss

@@ -125,3 +125,44 @@ limitations under the License.
         margin-top: 12px;
     }
 }
+
+.mx_LeftPanel_exploreAndFilterRow {
+    display: flex;
+
+    .mx_SearchBox {
+        flex: 1 1 0;
+        min-width: 0;
+    }
+}
+
+.mx_LeftPanel_explore {
+    flex: 0 0 40%;
+    overflow: hidden;
+    box-sizing: border-box;
+
+    .mx_AccessibleButton {
+        font-size: 14px;
+        margin: 9px;
+        margin-right: 0;
+        padding: 9px;
+        padding-left: 42px;
+        font-weight: 600;
+        color: $notice-secondary-color;
+        position: relative;
+        border-radius: 4px;
+
+        &::before {
+            cursor: pointer;
+            mask: url('$(res)/img/explore.svg');
+            mask-repeat: no-repeat;
+            mask-position: center center;
+            content: "";
+            left: 14px;
+            top: 10px;
+            width: 16px;
+            height: 16px;
+            background-color: $notice-secondary-color;
+            position: absolute;
+        }
+    }
+}

res/css/structures/_SearchBox.scss

@@ -14,6 +14,10 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
+.mx_SearchBox {
+    flex: 1 1 0;
+    min-width: 0;
+
     .mx_SearchBox_closeButton {
         cursor: pointer;
         background-image: url('$(res)/img/icons-close.svg');
@@ -23,3 +27,4 @@ limitations under the License.
         background-position: center;
         padding: 9px;
     }
+}

res/css/views/rooms/_RoomList.scss

@@ -27,10 +27,6 @@ limitations under the License.
     position: relative;
 }
 
-.mx_SearchBox {
-    flex: none;
-}
-
 /* hide resize handles next to collapsed / empty sublists */
 .mx_RoomList .mx_RoomSubList:not(.mx_RoomSubList_nonEmpty) + .mx_ResizeHandle {
     display: none;

res/img/explore.svg

@@ -0,0 +1,97 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg
+   xmlns:dc="http://purl.org/dc/elements/1.1/"
+   xmlns:cc="http://creativecommons.org/ns#"
+   xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
+   xmlns:svg="http://www.w3.org/2000/svg"
+   xmlns="http://www.w3.org/2000/svg"
+   xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
+   xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
+   width="21.124001"
+   height="14.896"
+   viewBox="0 0 21.124001 14.896"
+   version="1.1"
+   id="svg21"
+   sodipodi:docname="explore.svg"
+   inkscape:version="0.92.4 (unknown)">
+  <metadata
+     id="metadata25">
+    <rdf:RDF>
+      <cc:Work
+         rdf:about="">
+        <dc:format>image/svg+xml</dc:format>
+        <dc:type
+           rdf:resource="http://purl.org/dc/dcmitype/StillImage" />
+        <dc:title></dc:title>
+      </cc:Work>
+    </rdf:RDF>
+  </metadata>
+  <sodipodi:namedview
+     pagecolor="#ffffff"
+     bordercolor="#666666"
+     borderopacity="1"
+     objecttolerance="10"
+     gridtolerance="10"
+     guidetolerance="10"
+     inkscape:pageopacity="0"
+     inkscape:pageshadow="2"
+     inkscape:window-width="1920"
+     inkscape:window-height="1016"
+     id="namedview23"
+     showgrid="false"
+     fit-margin-top="0.5"
+     fit-margin-left="0.5"
+     fit-margin-right="0.5"
+     fit-margin-bottom="0.5"
+     inkscape:zoom="3.1891892"
+     inkscape:cx="-23.683763"
+     inkscape:cy="5.4480001"
+     inkscape:window-x="0"
+     inkscape:window-y="27"
+     inkscape:window-maximized="1"
+     inkscape:current-layer="svg21" />
+  <defs
+     id="defs15">
+    <filter
+       id="a"
+       width="1.118"
+       height="1.158"
+       x="-0.059"
+       y="-0.079000004"
+       filterUnits="objectBoundingBox">
+      <feOffset
+         dy="2"
+         in="SourceAlpha"
+         result="shadowOffsetOuter1"
+         id="feOffset2" />
+      <feGaussianBlur
+         in="shadowOffsetOuter1"
+         result="shadowBlurOuter1"
+         stdDeviation="16"
+         id="feGaussianBlur4" />
+      <feColorMatrix
+         in="shadowBlurOuter1"
+         result="shadowMatrixOuter1"
+         values="0 0 0 0 0 0 0 0 0 0.473684211 0 0 0 0 1 0 0 0 0.241258741 0"
+         id="feColorMatrix6" />
+      <feMerge
+         id="feMerge12">
+        <feMergeNode
+           in="shadowMatrixOuter1"
+           id="feMergeNode8" />
+        <feMergeNode
+           in="SourceGraphic"
+           id="feMergeNode10" />
+      </feMerge>
+    </filter>
+  </defs>
+  <g
+     transform="translate(-91.438,-120.552)"
+     id="g19"
+     style="fill:none;fill-rule:evenodd;stroke:#61708b;stroke-width:1.29999995;stroke-linecap:round;filter:url(#a)">
+    <path
+       d="m 98,122 h 13 m -13,6 h 13 m -13,6 h 13 M 93,122 h 0.5 m -0.5,6 h 0.5 m -0.5,6 h 0.5"
+       id="path17"
+       inkscape:connector-curvature="0" />
+  </g>
+</svg>

src/components/structures/LeftPanel.js

@@ -218,6 +218,7 @@ const LeftPanel = React.createClass({
         const TopLeftMenuButton = sdk.getComponent('structures.TopLeftMenuButton');
         const SearchBox = sdk.getComponent('structures.SearchBox');
         const CallPreview = sdk.getComponent('voip.CallPreview');
+        const AccessibleButton = sdk.getComponent('elements.AccessibleButton');
 
         const tagPanelEnabled = SettingsStore.getValue("TagPanel.enableTagPanel");
         let tagPanelContainer;
@@ -241,6 +242,15 @@ const LeftPanel = React.createClass({
             },
         );
 
+        let exploreButton;
+        if (!this.props.collapsed) {
+            exploreButton = (
+                <div className={classNames("mx_LeftPanel_explore", {"mx_LeftPanel_explore_hidden": this.state.searchFocused})}>
+                    <AccessibleButton onClick={() => dis.dispatch({action: 'view_room_directory'})}>{_t("Explore")}</AccessibleButton>
+                </div>
+            );
+        }
+
         const searchBox = (<SearchBox
             enableRoomSearchFocus={true}
             placeholder={ _t('Filter room names') }
@@ -259,7 +269,10 @@ const LeftPanel = React.createClass({
                 <aside className={"mx_LeftPanel dark-panel"} onKeyDown={ this._onKeyDown } onFocus={ this._onFocus } onBlur={ this._onBlur }>
                     <TopLeftMenuButton collapsed={ this.props.collapsed } />
                     { breadcrumbs }
+                    <div className="mx_LeftPanel_exploreAndFilterRow">
+                        { exploreButton }
                         { searchBox }
+                    </div>
                     <CallPreview ConferenceHandler={VectorConferenceHandler} />
                     <RoomList
                         ref={this.collectRoomList}

src/i18n/strings/en_EN.json

@@ -1474,6 +1474,7 @@
     "This homeserver does not support communities": "This homeserver does not support communities",
     "Failed to load %(groupId)s": "Failed to load %(groupId)s",
     "Filter room names": "Filter room names",
+    "Explore": "Explore",
     "Failed to reject invitation": "Failed to reject invitation",
     "This room is not public. You will not be able to rejoin without an invite.": "This room is not public. You will not be able to rejoin without an invite.",
     "Are you sure you want to leave the room '%(roomName)s'?": "Are you sure you want to leave the room '%(roomName)s'?",