Nested front-end-only EventHandler

Question

Bug introduced in 11.3 or earlier, and persisting through 12.0

CASE:4372001 confirmed

I am running into an issue trying to improve the performance of my user interface by moving as much of the state handling into the front-end as possible. It appears as though nesting EventHandler forces the kernel to be called, no matter what the action to be performed is.

The following examples illustrate what I am trying to achieve, and how it fails:

Single EventHandler, front-end-only state

This one works as intended: Clicking on the number toggles between 1 and 2, and no front-end <-> kernel communication occurs (verified with LinkSnooper)

DynamicModule[
 {open},
 EventHandler[
  PaneSelector[{True -> 1, False -> 2}, Dynamic@open],
  {
   "MouseClicked" :> FEPrivate`Set[open, FEPrivate`Not@open]
   }
  ]
 ]

Nested EventHandler, front-end-only state

This is the problematic case: Despite the PassEventsDown -> True, this doesn't work.

EventHandler[
 DynamicModule[
  {open},
  EventHandler[
   PaneSelector[{True -> 1, False -> 2}, Dynamic@open],
   {"MouseClicked" :> FEPrivate`Set[open, FEPrivate`Not@open]}
   ]
  ],
 {"MouseClicked" :> True},
 PassEventsDown -> True
 ]

Nested EventHandler, kernel-side state

This one works as expected, but needs to call the kernel:

EventHandler[
 DynamicModule[
  {open},
  EventHandler[
   PaneSelector[{True -> 1, False -> 2}, Dynamic@open],
   {"MouseClicked" :> (open = ! open)}
   ]
  ],
 {"MouseClicked" :> True},
 PassEventsDown -> True
 ]

Nested EventHandler, front-end-only state, using FE`Evaluate

This one is thanks to @Kuba, who noticed that the event action is sent to the kernel even though it shouldn't be. This means that adding FE`Evaluate can be used to make it work. The problem is of course that this still calls the kernel.

EventHandler[
 DynamicModule[
  {open},
  EventHandler[
   PaneSelector[{True -> 1, False -> 2}, Dynamic@open],
   {"MouseClicked" :> FE`Evaluate@FEPrivate`Set[open, FEPrivate`Not@open]}
   ]
  ],
 {"MouseClicked" :> True},
 PassEventsDown -> True
 ]

Question

Since WRI support already confirmed that this is a bug, I am only looking for a possible work-around. Is there any way to get nested, clickable boxes that work entirely in the front-end?

Answer 1

As confirmed by both WRI support and @ihojnicki, this is currently not possible. The front-end does not appear to be able to handle nested EventHandler expressions on its own.

Quoting WRI support: ([CASE:4372001])

It does appear that Nested EventHandler is not behaving properly when all computations are moved to front-end. I have forwarded an issue report to our developers with the information you provided. I also added your contact information to the issue report, so that you can be notified if the issue is resolved in a future version of Mathematica.

I have looked into alternative approaches but unfortunately have not been able to make them work for your specific issue.

For now, I have solved my problem by restructuring the GUI to not require nesting of EventHandlers. This way, one can preserve the responsiveness that comes with front-end only user-interfaces.