Jquery ui effects filename have changed. For example change

footer|combine_script require='jquery.effects.blind'

to

footer|combine_script require='jquery.ui.effect-blind'

We had backward naming compatibility for 2.5 but not for 2.6.

the file admin/themes/default/uplodify.jgrowl.css was moved to themes/default/js/plugins/jquery.jgrowl.css

use this line to include the file in your template :

{combine_css path="themes/default/js/plugins/jquery.jgrowl.css"}

no changes for the javascript file

Following http://piwigo.org/doc/doku.php?id=dev:changes_in_2.5#add_action_buttons

Themes have to handle two new template vars : $PLUGIN_INDEX_BUTTONS and $PLUGIN_PICTURE_BUTTONS
Plugins creators SHOULD NOT add <li> or <div> or <span> around their buttons, it's now handled by the theme.

Default implementation is

{if !empty($PLUGIN_INDEX_BUTTONS)}{foreach from=$PLUGIN_INDEX_BUTTONS item=button}<li>{$button}</li>{/foreach}{/if}

in index.tpl and

{if isset($PLUGIN_PICTURE_BUTTONS)}{foreach from=$PLUGIN_PICTURE_BUTTONS item=button}{$button}{/foreach}{/if}

in picture.tpl

TokenInput CSS was moved from admin/themes/default/style.css to themes/default/js/plugins/jquery.tokeninput.css. Needs to be explicitly imported with

{combine_css path="themes/default/js/plugins/jquery.tokeninput.css"}

allowCreation parameter was renamed into allowFreeTagging

SwitchBox are used for derivatives choices, sort options, LanguageSwitch, ThemeSwitch, etc.
Now the JS code is factorized in a function available in themes/default/js/switchbox.js
Usage :

{combine_script id='core.switchbox' load='async' require='jquery' path='themes/default/js/switchbox.js'}
{footer_script require='core.switchbox'}(SwitchBox=window.SwitchBox||[]).push("#sortOrderLink", "#sortOrderBox");{/footer_script}

The CSS is still provided by themes.

In template/menubar_categories.tpl, replace:

{'</ul></li>'|@str_repeat:$ref_level-$cat.LEVEL}

by

{'</ul></li>'|@str_repeat:($ref_level-$cat.LEVEL)}

Added a function translate_dec, replace:

{$pwg->l10n_dec('singular string', 'plural string', $value)}

by

{$value|@translate_dec:'singular string':'plural string'}

Many plugins add a new public page to Piwigo. Lacking of a documented way to do it, everybody was using his own method, causing display issues with some themes.

In Piwigo 2.6 we introduced a simple way to do it.

First define in loc_end_section_init trigger :

$page['body_id'] = 'whatever_you_want';
$page['is_external'] = true;

Then when you want to display the content of your page :

$template->set_filename('your_page', realpath(path to your template));
$template->assign_var_from_handle('CONTENT', 'your_page');

Check Skeleton plugin for complete example http://piwigo.org/svn/extensions/skeleton/trunk/include/public_events.inc.php

There a little modification of index.tpl : you need to add

{if !empty($CONTENT)}{$CONTENT}{/if}

in the main block of the page.

admin.php?page=batch_manager could be accessed with four URL parameters :
&cat=caddie
&cat=recent
&cat=###
&tag=###

This has been replaced by a single parameter filter :
&filter=prefilter-caddie
&filter=prefilter-last_import
&filter=album-##
&filter=tag-##

Now any prefilter (build-in or added by a plugin) can be accessed via this URL parameter. Build-in prefilters are : caddie, favorites, last_import, no_album, no_tag, duplicates, all_photos, no_virtual_album.