ui: Implement modal focus trap

ui/src/Layout.elm

@@ -37,6 +37,7 @@ import Html.Attributes
         , value
         )
 import Html.Events as Events
+import Modal
 import Route exposing (Route)
 
 
@@ -79,6 +80,8 @@ reset model =
 
 type Msg
     = ClearFlash
+    | ModalFocused Modal.Msg
+    | ModalUnfocused
     | OnMailboxNameInput String
     | OpenMailbox
     | ShowRecent Bool
@@ -94,6 +97,15 @@ update msg model session =
             , Cmd.none
             )
 
+        ModalFocused message ->
+            ( model
+            , Modal.updateSession message session
+            , Cmd.none
+            )
+
+        ModalUnfocused ->
+            ( model, session, Modal.resetFocusCmd (ModalFocused >> model.mapMsg) )
+
         OnMailboxNameInput name ->
             ( { model | mailboxName = name }
             , session
@@ -165,7 +177,7 @@ frame { model, session, activePage, activeMailbox, modal, content } =
                 ]
             ]
         , div [ class "navbar-bg" ] [ text "" ]
-        , frameModal modal
+        , Modal.view (ModalUnfocused |> model.mapMsg) modal
         , div [ class "page" ] (errorFlash model session.flash :: content)
         , footer []
             [ div [ class "footer" ]
@@ -178,18 +190,6 @@ frame { model, session, activePage, activeMailbox, modal, content } =
         ]
 
 
-frameModal : Maybe (Html msg) -> Html msg
-frameModal maybeModal =
-    case maybeModal of
-        Just modal ->
-            div [ class "modal-mask" ]
-                [ div [ class "modal well" ] [ modal ]
-                ]
-
-        Nothing ->
-            text ""
-
-
 errorFlash : Model msg -> Maybe Session.Flash -> Html msg
 errorFlash model maybeFlash =
     let

ui/src/Modal.elm

@@ -0,0 +1,58 @@
+module Modal exposing (Msg, resetFocusCmd, updateSession, view)
+
+import Browser.Dom as Dom
+import Data.Session as Session exposing (Session)
+import Html exposing (Html, div, span, text)
+import Html.Attributes exposing (class, id, tabindex)
+import Html.Events exposing (onFocus)
+import Task
+
+
+type alias Msg =
+    Result Dom.Error ()
+
+
+{-| Creates a command to focus the modal dialog.
+-}
+resetFocusCmd : (Msg -> msg) -> Cmd msg
+resetFocusCmd resultMsg =
+    Task.attempt resultMsg (Dom.focus domId)
+
+
+{-| Updates a Session with an error Flash if the resetFocusCmd failed.
+-}
+updateSession : Msg -> Session -> Session
+updateSession result session =
+    case result of
+        Ok () ->
+            session
+
+        Err (Dom.NotFound missingDomId) ->
+            let
+                flash =
+                    { title = "DOM element not found"
+                    , table = [ ( "Element ID", missingDomId ) ]
+                    }
+            in
+            Session.showFlash flash session
+
+
+view : msg -> Maybe (Html msg) -> Html msg
+view unfocusedMsg maybeModal =
+    case maybeModal of
+        Just modal ->
+            div [ class "modal-mask" ]
+                [ span [ onFocus unfocusedMsg, tabindex 0 ] []
+                , div [ id domId, class "modal well", tabindex -1 ] [ modal ]
+                , span [ onFocus unfocusedMsg, tabindex 0 ] []
+                ]
+
+        Nothing ->
+            text ""
+
+
+{-| DOM ID of the modal dialog.
+-}
+domId : String
+domId =
+    "modal-dialog"

ui/src/Page/Mailbox.elm

@@ -1,6 +1,7 @@
 module Page.Mailbox exposing (Model, Msg, init, load, subscriptions, update, view)
 
 import Api
+import Browser.Dom as Dom
 import Data.Message as Message exposing (Message)
 import Data.MessageHeader exposing (MessageHeader)
 import Data.Session as Session exposing (Session)
@@ -51,6 +52,7 @@ import Html.Events as Events
 import HttpUtil
 import Json.Decode as D
 import Json.Encode as E
+import Modal
 import Route
 import Task
 import Time exposing (Posix)
@@ -174,6 +176,7 @@ type Msg
     | PurgedMailbox (Result HttpUtil.Error ())
     | OnSearchInput String
     | Tick Posix
+    | ModalFocused Modal.Msg
 
 
 update : Msg -> Model -> ( Model, Cmd Msg )
@@ -262,6 +265,11 @@ update msg model =
         MessageBody bodyMode ->
             ( { model | bodyMode = bodyMode }, Cmd.none )
 
+        ModalFocused message ->
+            ( { model | session = Modal.updateSession message model.session }
+            , Cmd.none
+            )
+
         OnSearchInput searchInput ->
             updateSearchInput model searchInput
 
@@ -293,13 +301,13 @@ update msg model =
                     ( model, Cmd.none )
 
         PurgeMailboxPrompt ->
-            ( { model | promptPurge = True }, Cmd.none )
+            ( { model | promptPurge = True }, Modal.resetFocusCmd ModalFocused )
 
         PurgeMailboxCanceled ->
             ( { model | promptPurge = False }, Cmd.none )
 
         PurgeMailboxConfirmed ->
-            updatePurge model
+            updateTriggerPurge model
 
         PurgedMailbox (Ok _) ->
             ( model, Cmd.none )
@@ -358,8 +366,10 @@ updateMessageResult model message =
             )
 
 
-updatePurge : Model -> ( Model, Cmd Msg )
-updatePurge model =
+{-| Updates model and triggers commands to purge this mailbox.
+-}
+updateTriggerPurge : Model -> ( Model, Cmd Msg )
+updateTriggerPurge model =
     let
         cmd =
             Cmd.batch

ui/src/main.css

@@ -223,6 +223,10 @@ h3 {
   padding: 10px !important;
 }
 
+.modal:focus {
+  outline: none;
+}
+
 /** BUTTONS */
 
 .button-bar {