[Plugin: Post UI Tabs] support for nested shortcodes?

Whilst that’s certainly an option, you should note that the data a filter passes to functions hooking on can only be passed back the data from the first argument, so using your above example, regardless of what you may change in $content, that data would not be getting passed back to the plugin.

An option would be would be pass an array of the data into the hook, eg.

$this->has_tabs = apply_filters( 'put_decide_has_tabs', array( $this->has_tabs, $content ) );

Though i’m inclined to say that if you want to hook the post content and determine whether to enable/disable tabs, you should do so with your own filter on the_content and i’d be happy to add a filter to set the has_tabs variable, so you can more easily set that via a filter instead of bringing the $Post_UI_Tabs object into scope and explicitly setting the has_tabs property.

$this->has_tabs = (bool) apply_filters( 'put_decide_has_tabs', $this->has_tabs );

If you wanted to run some conditional logic over the post content before setting that hook you could run two filters, something along the lines of..

// Hook at priority 6, plugin runs filter at priority 7
add_filter( 'the_content', 'my_filter_on_the_content', 6 );

// Assuming you want tabs disabled unless logic in filter is true
$set_tabs = false;

function my_filter_on_the_content( $content ) {

	global $set_tabs;
	// Do condition logic to determine whether to enable has_tabs
	$set_tabs = true;

	return $content;
}
if( $set_tabs )
	add_filter( 'put_decide_has_tabs', '__return_true' );

The logic can be inversed if you want to have tabs on, but conditionally turn them off.

I’ll add the filter to the next update anyway, it definitely makes sense to have users set that variable via a filter instead of messing around with the class object.