I have created a lightning component and assigned it to a custom case action. How to add this action to header near standard buttons.
Activity buttons and they are more like tabs in LEX and will stay in the Activities standard component - you should see it in the app builder:
Again - this is the Activities standard component - you cannot change it so you cannot remove the create new textbox and the add button.
Case Comments is a related object list - you can put it wherever you want with the app builder - just drag the Related List component and then select the object:
Please refer to this: https://help.salesforce.com/articleView?id=actions_in_lex.htm&type=0
In Lightning Experience, actions display in the Global Actions menu in the header, on related lists, on list view items, and in several places on a record page. Where they appear on a record page depends on the action’s type.
[...] The page-level action menu in the record’s highlights panel (1) contains: [...] Global and object-specific quick actions, except for those related to creating tasks, creating events, and logging calls. [...]
The Activity tab (2) contains Create a Record quick actions that point to the Event and Task objects. It also contains Log A Call actions and Send Email actions.
It's a built-in Lightning page behaviour. If you have a Quick Action that creates an Activity or sends an E-mail, it winds up in that tab instead of the button area. They just don't give you any choice in the matter.
Best Answer
There is a catch in case of cases and work orders objects when it comes to quick actions. In the article under Actions on record pages there is note that says
For other objects, you can edit page in the Lightning Experience and then select Salesforce1 and Lightning Experience Actions from the selection panel. The panel states
Once you click
override the predefined actionslink, it will give you available actions for SF1 and lightning to use. And then position your lightning-action in your desired sequence, as shown below.I worked on the Account object for this example, it should be no exception for other objects, and this is what I got
It should be noted that you can't add more than three (3) viewable lightning-actions on the page, as shown in the picture above in green squareYou can display upto 10 actions on page layout, but other action will available when you click the drop-down next to the last button on the right. There is an idea on this to let users display more Action Buttons on Lightning Highlights Panel, you can upvote that if it affects you.Edit
I have just come across this Page Layouts in Lightning Experience trailhead and it says
I just edited the highlights panel using page layout editor, strange. May be this trailhead content is old, if someone can put some light on this.