On every JIRA issue page you can find new "Slack" tab in the Activity section (near Comments). If the issue you are looking at has a dedicated channel assigned you will be able to quickly join the Slack discussion directly from JIRA.
After opening the "Slack" tab you will see up to 100 most recent messages on that channel.
Messages may look slightly different than in you Slack client because this view uses a simplified message formatter. Please let us know
if the simplified formatter does a really weird thing or is missing support of an important feature.
Moreover you can join Slack conversation directly from here as the input field below the last message lets you send messages back to that channel.
Due to technical limitation messages sent by others will not appear immediately. Selecting an input field and typing inside this field is likely to trigger refresh. At last you can manually trigger refresh by clicking the small "Refresh now" link below the last message.