TYPO3 Core

To easily test the hook add next lines to your typo3conf/AdditionalConfiguration.php

            $GLOBALS['TYPO3_CONF_VARS']['SC_OPTIONS']['Backend\Template\Components\ButtonBar']['getButtonsHook']['HookTest'] =
            function($params, TYPO3\CMS\Backend\Template\Components\ButtonBar $buttonBar) {
                $buttons = $params['buttons'];
//                if (\TYPO3\CMS\Core\Utility\GeneralUtility::_GP('M') === 'web_list') {
                    $iconFactory = \TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance(\TYPO3\CMS\Core\Imaging\IconFactory::class);
                    $button = $buttonBar->makeLinkButton();
                    $button->setShowLabelText(true);
                    $button->setIcon($iconFactory->getIcon('actions-system-backend-user-emulate', \TYPO3\CMS\Core\Imaging\Icon::SIZE_SMALL));
                    $button->setTitle('getButtons hook test');
                    $button->setOnClick('alert("Hook works");return false;');

                    $buttons[\TYPO3\CMS\Backend\Template\Components\ButtonBar::BUTTON_POSITION_LEFT][1][] = $button;
//                }
                return $buttons;
            };

Patch set 2 for branch master of project Packages/TYPO3.CMS has been pushed to the review server.
It is available at https://review.typo3.org/44569