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Here's a workaround to keep backword compatibility with older b2evo versions

var $group = 'My group name'; // for b2evo < 5.0

function PluginInit( & $params )
{
	global $app_version;

	if( version_compare( $app_version, '5' ) > 0 )
	{
		$this->group = 'rendering';
	}

After the upgrade, an admin has to modify each plugin and delete $apply_rendering string, and then manually edit "Apply rendering to posts" setting in each single blog.

Since there's no docs describing this "feature" I had to use an old trial and error method. It looks like you don't need to delete $apply_rendering property. However you have to make sure you set $group = 'rendering'; to make your plugin appear in Blogs > Plugins page where you still have to select rendering method for each individual blog.

What if I want to keep my plugins in a separate group as it was done before? Anyway, it's clearly a feature that makes older rendering plugins useless on upgrade.

Yes, we clearly don't want to restrict new feature to accommodate old plugins.

However, we take as many precautions as possible so as to only introduce these kind of changes from one major version to the next, in this case from 4.x to 5.x

Why don't you simply apply plugins rendering settings to each blog on upgrade

1. Get installed rendering plugins
2. Read their rendering method
3. Loop through blogs and apply that method

Really, there's no reason to break all rendering plugins if it only takes 15 minutes to add a few lines to the upgrade script

I thought "group" variable was only meant for grouping plugins virtually like taxonomy - no actual functionality. And that's how we all used it before, sam2kb, Afwas, Yabba and myself to put them under "AM_" "Amazind_discoveries" "b2evoru" etc..

Anyway thanks for the heads up, so we should find another way to "group" our plugins :D