DavidUnzue
commented
8 years ago I just realized this is actually caused by other field extension, kirby-structure-tabs, which is based on yours and I had installed.
Kirby-structure-tabs is loading the same javascript as kirby-tabs-field when a modal of a structure field opens, regardless of a structure-tab field exists in there or not. This causes the first tab to be selected, both in the structure-tab field and the tab field.
So the fault is not yours but from kirby-structure-tabs field.
When adding or editing an entry on Kirby's structure field within any tab but the first one, the opening modal causes the first tab to be selected too (it gets the "current" css class added), so that at the end, both tabs are selected and open at the same time. Thus the content of both tabs shows up and the interface is broken.