Allow empty menus to activate in menu bar

[sdirix]

Hi! I'm working on migrating to Lumino in Theia. We’ve previously relied on PhosphorJS, and decided to transition to Lumino.

Problem

We implemented a DynamicMenuWidget whose core functionality involves dynamically calculating its menu items just before being shown. This worked in PhosphorJS

However, we’ve encountered an issue with Lumino due to a change introduced in #607. Specifically, this check in the MenuBar prevents our menus from activating:

    // An empty menu cannot be active
    if (value > -1 && this._menus[value].items.length === 0) {
      value = -1;
    }

As a result, Theia’s dynamic menus are never activated and therefore never displayed.

Workarounds

We’ve explored a few workarounds:

• Patching Lumino locally via patch-package to remove the check, but this requires maintenance with every new Lumino release.
• Subclassing MenuBar and overriding the activeIndex setter to bypass the check, but this requires duplicating the setter code and working around type checks on private fields.

Both approaches feel brittle and suboptimal for long-term maintenance.

Proposed Solution?

Would you be open to changes that make the MenuBar more flexible for downstream adopters like us? If so, we’d appreciate your input on the best way forward. Some potential approaches:

• Removing the check entirely if it’s not critical for some other use cases
• Making private fields protected in MenuBar to allow subclassing without type workarounds.
• Moving the check into a separate method that subclasses can override.

Alternatively, if you have suggestions for a better way to implement dynamic menus in Theia that align more closely with Lumino's current design, I’d be very interested to hear your thoughts.

Thanks for all your work on Lumino—it’s a fantastic library, and we look forward to using it in Theia!

[krassowski]

CC @gabalafou as author of #607.

[jtpio]

Thanks @sdirix for opening the issue!

Cool to see Theia updating to Lumino! Agree it would be much better if Theia could just use the Lumino packages without having to maintain a patch 👍

• Removing the check entirely if it’s not critical for some other use cases

Looking at the PR there does not seem to be any discussion about that check. But maybe @gabalafou or @fcollonval (reviewer) will know more.

• Making private fields protected in MenuBar to allow subclassing without type workarounds.

This is indeed an approach that has been taken before to allow for easier subclassing. Maybe one downside to this would be the protected fields increasing the API surface.

• Moving the check into a separate method that subclasses can override.

Maybe this could be the less disruptive one, while allowing for easier customization downstream?

If you would you like to start exploring this in a draft PR, that would be great, thanks!

[fcollonval]

I second @jtpio. A good way would be to create a protected validateActiveIndex method that you could override.

[sdirix]

Thanks for the feedback 👍 . Sounds good, I will open such a PR