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Please add an option to replace tab with spaces. It looks like that was configurable in an older release, but currently there is no setting avilable.
Select one of the "with spaces only" options at Options -> Editor -> Indentation mode
Oh... didn't look for it in that place. Perhaps moving it to the place where tab width is configured would make it easier to find.
With TeXstudio 2.8.6 this option is ineffective
There is still an option Config -> Editor -> Replace Tab by Whitespace. However due to changes in the indentation code this was only taken into account if there was a selection. This is now fixed: hg 4781 (48581a73ca35)
Config -> Editor -> Replace Tab by Whitespace
As a workaround you could select a character on the desired line and then hit <tab>.