Engine: fix exception when unfocusing too much

Author: panglesd

src/engine/actions.ml

@@ -53,7 +53,9 @@ module AttributeActions = struct
   let unfocus window elem =
     let action _elem =
       let> coord = State.Focus.pop () in
-      Window.move window coord ~delay:1.0
+      match coord with
+      | None -> UndoMonad.return ()
+      | Some coord -> Window.move window coord ~delay:1.0
     in
     act_on_elem "unfocus-at-unpause" action elem
 

src/engine/state.ml

@@ -25,7 +25,9 @@ module Focus = struct
     UndoMonad.return ~undo (Stack.push c stack)
 
   let pop () =
-    let value = Stack.pop stack in
-    let undo () = Fut.return @@ Stack.push value stack in
+    let value = Stack.pop_opt stack in
+    let undo () =
+      Fut.return @@ Option.iter (fun v -> Stack.push v stack) value
+    in
     UndoMonad.return ~undo value
 end

src/engine/state.mli

@@ -5,5 +5,5 @@ val incr_step : unit -> unit UndoMonad.t
 
 module Focus : sig
   val push : Coordinates.window -> unit UndoMonad.t
-  val pop : unit -> Coordinates.window UndoMonad.t
+  val pop : unit -> Coordinates.window option UndoMonad.t
 end