In last version of Suite - 7.38 there is new bug that validate function of Combo does not return result of validation at all, so it is always undefined. In previous versions there was correct.
Example is there: https://snippet.dhtmlx.com/a3bv4w2d
Your product is completely untrustworthy.
Hello @Przemyslaw_Niedziela,
Thank you for the detailed description of the issue, I sent it to the dev team, and they will work on a fix.
Kind regards,
We have fixed your reported problem in the latest dhx.Suite update (v.7.3.9).
Now the validation should work properly.
You can test it in your original snippet.