This page of dotPeek options allows you to configure code viewer tabs.
Allow new files to be open in the preview tab
If this checkbox is enabled, all new documents are displayed in the preview tab, which only keeps the open document until you open a new one.
Insert new tabs to the right of existing tabs
Use this checkbox to configure whether to open new code viewer tabs at the left or at the right.
Show pin button in unpinned tabs
When this checkbox is enabled, you will see the pin button tab. You can click it to pin/unpin the tab.
Even if the pin button is hidden, you can right-click the tab and choose Pin/Unpin Tab.
Enable tab colors to indicate code presentation mode
If this checkbox is selected, each tab will be colored according to its presentation mode: