Fix double-check of task list

Since commit 12a76fee33, we had a potential
bug on the event mechanism:

It introduced (besides the checkTasks method itself) a second run of the
checkTasks() method. In the second run, however, scheduled events were
not taken into consideration in how long to wait for input events
afterwards.

So when the after the first run of checkTasks() there were new scheduled
tasks added to the task queue, they were not properly scheduled and
and depended on an already existing scheduled event or an input event
to trigger.

This might have led to unexpected order of execution (though the order
is not guaranteed by the task scheduling anyway!) or to events triggering
not at all until the next input event.

frontend/ui/uimanager.lua

@@ -69,6 +69,7 @@ local UIManager = {
     _running = true,
     _window_stack = {},
     _execution_stack = {},
+    _execution_stack_dirty = false,
     _dirty = {},
     _zeromqs = {},
 }
@@ -232,6 +233,7 @@ end
 -- schedule an execution task
 function UIManager:schedule(time, action)
     table.insert(self._execution_stack, { time = time, action = action })
+    self._execution_stack_dirty = true
 end
 
 -- schedule task in a certain amount of seconds (fractions allowed) from now
@@ -370,7 +372,8 @@ function UIManager:checkTasks()
             end
         end
     until all_tasks_checked
-    return wait_until
+    self._execution_stack_dirty = false
+    return wait_until, now
 end
 
 -- repaint dirty widgets
@@ -477,26 +480,31 @@ end
 function UIManager:run()
     self._running = true
     while self._running do
-        local now = { util.gettime() }
-        local wait_until = self:checkTasks()
-
-        --DEBUG("---------------------------------------------------")
-        --DEBUG("exec stack", self._execution_stack)
-        --DEBUG("window stack", self._window_stack)
-        --DEBUG("dirty stack", self._dirty)
-        --DEBUG("---------------------------------------------------")
-
-        -- stop when we have no window to show
-        if #self._window_stack == 0 then
-            DEBUG("no dialog left to show")
-            self:quit()
-            return nil
-        end
+        local wait_until, now
+        -- run this in a loop, so that paints can trigger events
+        -- that will be honored when calculating the time to wait
+        -- for input events:
+        repeat
+            wait_until, now = self:checkTasks()
+
+            --DEBUG("---------------------------------------------------")
+            --DEBUG("exec stack", self._execution_stack)
+            --DEBUG("window stack", self._window_stack)
+            --DEBUG("dirty stack", self._dirty)
+            --DEBUG("---------------------------------------------------")
+
+            -- stop when we have no window to show
+            if #self._window_stack == 0 then
+                DEBUG("no dialog left to show")
+                self:quit()
+                return nil
+            end
 
-        self:checkTasks()
+            self:repaint()
+        until not self._execution_stack_dirty
 
         -- wait for next event
-        -- note that we will skip that if in the meantime we have tasks that are ready to run
+        -- note that we will skip that if we have tasks that are ready to run
         local input_event = nil
         if not wait_until then
             if #self._zeromqs > 0 then
@@ -509,7 +517,7 @@ function UIManager:run()
                     end
                 end
             else
-                -- no pending task, wait endlessly
+                -- no pending task, wait without timeout
                 input_event = Input:waitEvent()
             end
         elseif wait_until[1] > now[1]