yii\bootstrap\ButtonDropdown splitButton gets same options as primary button

[yii-bot]

This issue has originally been reported by @ASlatius at yiisoft/yii2#6720.
Moved here by @cebe.

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When rendering with 'split'=>true, the dropdown-toggle is rendered with a second button. This button gets the same options as the primary buttons.
This is getting problematic when you try and set options for JavaScript usage like 'id' and 'onclick'.

[ruturajmaniyar]

You can simply change on your code and resolved this issue.
In ButtonDropDown.php line number : 109, you can find below code.

if ($this->split) {
            $options = $this->options;
            $this->options['data-toggle'] = 'dropdown';
            Html::addCssClass($this->options, ['toggle' => 'dropdown-toggle']);
            unset($options['id']);
            $splitButton = Button::widget([
                'label' => '<span class="caret"></span>',
                'encodeLabel' => false,
                'options' => $this->options,
                'view' => $this->getView(),
            ]);
        } else {
            $label .= ' <span class="caret"></span>';
            $options = $this->options;
            Html::addCssClass($options, ['toggle' => 'dropdown-toggle']);
            $options['data-toggle'] = 'dropdown';
            $splitButton = '';
        }

you can change in above code, in "ELSE" part only and solved "splitButton" option issue.

else {
            $label .= ' <span class="caret"></span>';
            $options = $this->options;
            $options['data-toggle'] = 'dropdown';
            Html::addCssClass($options, ['toggle' => 'dropdown-toggle']);
            unset($options['id']);
            $splitButton = '';
        }

Here issue is that 'data-toggle' option want to add before "html" and need to unset "$options['id']" after html is ready.