There is no access key set for the summary field on the Wikibase undo page, despite the title saying the usual b works. The current HTML code is the following (reformatted for clarity):
<div id='wpSummaryLabel' class='mw-summary oo-ui-layout oo-ui-labelElement oo-ui-fieldLayout oo-ui-fieldLayout-align-top'> <div class='oo-ui-fieldLayout-body'> <span class='oo-ui-fieldLayout-header'> <label for='ooui-php-1' class='oo-ui-labelElement-label'>Summary (will be appended to an automatically generated summary):</label> </span> <div class='oo-ui-fieldLayout-field'> <div aria-disabled='false' class='oo-ui-widget oo-ui-widget-enabled oo-ui-inputWidget oo-ui-textInputWidget oo-ui-textInputWidget-type-text oo-ui-textInputWidget-php'> <input type='text' tabindex='0' aria-disabled='false' title='Enter a short summary [b]' name='wpSummary' value='' maxlength='200' spellcheck='true' id='ooui-php-1' class='oo-ui-inputWidget-input' /> <span class='oo-ui-iconElement-icon oo-ui-iconElement-noIcon'></span> <span class='oo-ui-indicatorElement-indicator oo-ui-indicatorElement-noIndicator'></span> </div> </div> </div> </div>
The <input> element has no accesskey attribute at all, but should be accesskey='b' to follow the convention. The current situation (especially as the tabindex is set to 0) means that without using mouse, one has to tab through several elements to reach the summary field.