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.