I’m using it as a VST in Reaper 5.5. The same behavior in standalone SD3.
In Grid Editor, Quantize button grays out upon subsequent selections via clicking on instrument name.
1) Open Grid Editor
2) Select “Kick” in left column to select all kick hits and click on quantize. It quantizes.
3) Select “Snare” in left column to select all snare hits. Quantize button is grayed out.
If I then manually select the snare hits, the Quantize button reactivates.
If I close the Grid Editor and reopen it from the song track block, it reactivates.
OSX 10.10.5, Reaper 5.5, SD3 3.01
