When you use the operation "Duplicate Line" (found in the menu: Edit->Line Operations->Duplicate Line) the selected text is duplicated, but the selected text is increased to include the duplicated line.
The expected behaviour should be: the selected text stays the same and for the lines to be placed below the selected text. Please see image.
TeXstudio 2.12.6 (hg 6632:7777b2b8a906)
Using Qt Version 5.9.1, compiled with Qt 5.9.1 R
fixed in repository
This bug is back in version: TeXstudio 2.12.6 (hg 6632:7777b2b8a906) -- Using Qt Version 5.9.1, compiled with Qt 5.9.1 R