Fix cursor blinking problem

The problem was that when the editor had no events the cursor was not
blinking because the event loop was blocking on wait_event.

Now we no longer calls wait_event without a timeout if the windows has
the focus. When the window has the focus the timeout is set to 1 / fps
so that the cursor can blinks.

In addition we react to the "focus lost" event to ensure the documents
are redrawn without the cursor.

data/core/init.lua

@@ -378,6 +378,8 @@ function core.on_event(type, ...)
         core.root_view:open_doc(doc)
       end
     end
+  elseif type == "focuslost" then
+    core.root_view:on_focus_lost(...)
   elseif type == "quit" then
     core.quit()
   end
@@ -487,8 +489,13 @@ function core.run()
       -- do not wait of events at idle_iterations = 1 to give a chance at core.step to run
       -- and set "redraw" flag.
       if idle_iterations > 1 then
+        if system.window_has_focus() then
+          -- keep running even with no events to make the cursor blinks
+          system.wait_event(1 / config.fps)
+        else
           system.wait_event()
         end
+      end
     else
       idle_iterations = 0
     end

data/core/rootview.lua

@@ -504,6 +504,11 @@ function RootView:on_text_input(...)
 end
 
 
+function RootView:on_focus_lost(...)
+  -- We force a redraw so documents can redraw without the cursor.
+  core.redraw = true
+end
+
 function RootView:update()
   copy_position_and_size(self.root_node, self)
   self.root_node:update()

src/api/system.c

@@ -61,6 +61,10 @@ top:
         lua_pushstring(L, "exposed");
         return 1;
       }
+      if (e.window.event == SDL_WINDOWEVENT_FOCUS_LOST) {
+        lua_pushstring(L, "focuslost");
+        return 1;
+      }
       /* on some systems, when alt-tabbing to the window SDL will queue up
       ** several KEYDOWN events for the `tab` key; we flush all keydown
       ** events on focus so these are discarded */