I have installed the modifications described in thread 33423 in order to be able to price products based on their dimensions (width & height). This works fine.

Afterwards, I have installed the Zen Lightbox mod and it also works fine.

Note however that installation of the Zen Lightbox seems to have side effects on the modifications performed based on thread 33423 :
the computation of the price based on width and height of the products still works fine , but the fact that the customer is obliged to enter a width and an height for the product before clicking on 'add to cart' is no longer honored.
(for info, the modification proposed by thread 33423 uses Pricing by Letter; through Admin, this attribute is defined as being mandatory, in order to force the customer to enter a width and a height).

I do not understand what goes wrong because this checking is done in /includes/functions/functions_lookups.php (line 438 in ZC 1.3.7) and this module is not affected by the Zen Lightbox mod.

Any ideas about what goes wrong?