When using the text Editor (Code Editor) in Vivado 2016.3 or 2016.4, I can select a block of code by either clicking the Column Selection Mode icon or holding down the Ctrl key and dragging the cursor over the block of text.
This block of text can be copied, cut, or deleted.
However, if I attempt to insert a column of spaces, tabs or another character, it overwrites the characters in front of each row of the selected column rather than inserting the text in the location I select.
This issue was discovered in Vivado 2016.3 and will be fixed in Vivado 2017.1.
The work-around is to select and copy a block that you would like to insert (for example, a column of white space) and then insert that column where you want it to be.
This method requires an additional step, but can be made to work.