Prompt user to leave rooms/subspaces in a space when leaving space

res/css/_components.scss

@@ -86,6 +86,7 @@
 @import "./views/dialogs/_IncomingSasDialog.scss";
 @import "./views/dialogs/_InviteDialog.scss";
 @import "./views/dialogs/_KeyboardShortcutsDialog.scss";
+@import "./views/dialogs/_LeaveSpaceDialog.scss";
 @import "./views/dialogs/_ManageRestrictedJoinRuleDialog.scss";
 @import "./views/dialogs/_MessageEditHistoryDialog.scss";
 @import "./views/dialogs/_ModalWidgetDialog.scss";

res/css/views/dialogs/_AddExistingToSpaceDialog.scss

@@ -49,30 +49,6 @@ limitations under the License.
             font-weight: $font-semi-bold;
             line-height: $font-15px;
         }
-
-        .mx_AddExistingToSpace_entry {
-            display: flex;
-            margin-top: 12px;
-
-            // we can't target .mx_BaseAvatar here as it'll break the decorated avatar styling
-            .mx_DecoratedRoomAvatar {
-                margin-right: 12px;
-            }
-
-            .mx_AddExistingToSpace_entry_name {
-                font-size: $font-15px;
-                line-height: 30px;
-                flex-grow: 1;
-                overflow: hidden;
-                white-space: nowrap;
-                text-overflow: ellipsis;
-                margin-right: 12px;
-            }
-
-            .mx_Checkbox {
-                align-items: center;
-            }
-        }
     }
 
     .mx_AddExistingToSpace_section_spaces {
@@ -279,3 +255,27 @@ limitations under the License.
         display: contents;
     }
 }
+
+.mx_AddExistingToSpace_entry {
+    display: flex;
+    margin-top: 12px;
+
+    // we can't target .mx_BaseAvatar here as it'll break the decorated avatar styling
+    .mx_DecoratedRoomAvatar {
+        margin-right: 12px;
+    }
+
+    .mx_AddExistingToSpace_entry_name {
+        font-size: $font-15px;
+        line-height: 30px;
+        flex-grow: 1;
+        overflow: hidden;
+        white-space: nowrap;
+        text-overflow: ellipsis;
+        margin-right: 12px;
+    }
+
+    .mx_Checkbox {
+        align-items: center;
+    }
+}

res/css/views/dialogs/_LeaveSpaceDialog.scss

@@ -0,0 +1,110 @@
+/*
+Copyright 2021 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+.mx_LeaveSpaceDialog_wrapper {
+    .mx_Dialog {
+        display: flex;
+        flex-direction: column;
+        padding: 24px 32px;
+    }
+}
+
+.mx_LeaveSpaceDialog {
+    width: 440px;
+    display: flex;
+    flex-direction: column;
+    flex-wrap: nowrap;
+
+    &.mx_LeaveSpaceDialog_hasChildren {
+        height: 500px;
+    }
+
+    .mx_Dialog_content {
+        flex-grow: 1;
+        margin: 0 0 24px;
+        color: $tertiary-fg-color;
+        overflow-y: auto;
+
+        .mx_Dropdown_input {
+            border-radius: 8px;
+            border-color: $input-border-color;
+        }
+
+        .mx_Dropdown_option {
+            font-size: $font-15px;
+        }
+
+        .mx_SearchBox {
+            // To match the space around the title
+            margin: 0 0 15px 0;
+            flex-grow: 0;
+            border-radius: 8px;
+        }
+
+        .mx_LeaveSpaceDialog_noResults {
+            display: block;
+            margin-top: 24px;
+        }
+
+        .mx_LeaveSpaceDialog_section {
+            margin: 16px 0;
+
+            .mx_Dropdown + span {
+                display: inline-block;
+                margin-top: 8px;
+            }
+        }
+
+        .mx_LeaveSpaceDialog_section_warning {
+            position: relative;
+            border-radius: 8px;
+            margin: 12px 0;
+            padding: 12px 8px 12px 42px;
+            background-color: $header-panel-bg-color;
+
+            font-size: $font-12px;
+            line-height: $font-15px;
+            color: $secondary-fg-color;
+
+            &::before {
+                content: '';
+                position: absolute;
+                left: 10px;
+                top: calc(50% - 8px); // vertical centering
+                height: 16px;
+                width: 16px;
+                background-color: $secondary-fg-color;
+                mask-repeat: no-repeat;
+                mask-size: contain;
+                mask-image: url('$(res)/img/element-icons/room/room-summary.svg');
+                mask-position: center;
+            }
+        }
+
+        > p {
+            color: $primary-fg-color;
+        }
+    }
+
+    .mx_Dialog_buttons {
+        margin-top: 20px;
+
+        .mx_Dialog_primary {
+            background-color: $notice-primary-color !important; // override default colour
+            border-color: $notice-primary-color;
+        }
+    }
+}

src/components/views/dialogs/AddExistingToSpaceDialog.tsx

@@ -47,7 +47,7 @@ interface IProps extends IDialogProps {
     onCreateRoomClick(space: Room): void;
 }
 
-const Entry = ({ room, checked, onChange }) => {
+export const Entry = ({ room, checked, onChange }) => {
     return <label className="mx_AddExistingToSpace_entry">
         { room?.isSpaceRoom()
             ? <RoomAvatar room={room} height={32} width={32} />