basit çe html olarak denedim, döngü içerisine koymadan
<div style="overflow-y: scroll;">
<input type="checkbox" name="topic[]" value="$topicid"> $topics<br>
<input type="checkbox" name="topic[]" value="$topicid"> $topics<br>
<input type="checkbox" name="topic[]" value="$topicid"> $topics<br>
<input type="checkbox" name="topic[]" value="$topicid"> $topics<br>
<input type="checkbox" name="topic[]" value="$topicid"> $topics<br>
<input type="checkbox" name="topic[]" value="$topicid"> $topics<br>
<input type="checkbox" name="topic[]" value="$topicid"> $topics<br>
</div>
kaydırma çubuğu çıkmadı