Required field inside inactive tab breaks page saving

I have defined a custom blueprint for calendar event pages that uses tabs for organizing the various input fields. It works nicely, except for this one thing where a required field (event date in this case) on a tab that is currently not active will prevent saving the page without any error or other type of feedback.

A quick search led me to #1393, which appears to be about the same underlying issue. I'm also seeing a similar error: `An invalid form control with name='data[header][dates][start_date]' is not focusable.`.

A minimal version of the field definition looks as follows:

```yaml
data_tabs:
type: tabs
fields:
header.event:
type: tab
title: Event info
fields:
.name:
type: text
label: Name
validate:
required: true
header.dates:
type: tab
title: Dates
fields:
.start_date:
type: date
label: Start date
validate:
required: true
```