There is just under 9.7 ms of data to the left of the peak, if you type a value of 9.6 or lower into the left window you will be able to adjust it. I'll make a change to ensure REW sets the window to the correct maximum allowed in this circumstance.
Ok, now that works with the keyboard input ;-) Thanks.
But now appears another bug.
I can choose left window size smaller than 1 ms, e.g. 0.0 ms, and also "Apply" it, but as soon as I choose another file an come back to the previous file (with the 0.0 ms window) again, it says 1.0 ms in the "IR options" - but the Curve stays the same as with 0.0 ms. Same if I save the measurement and open it again later.
That's as confusing as my explanation ;-) - but I guess you guys understand what I mean.