This functionality has changed from Glyphs 2. If your intention is to shrink by 10%, then enter 90 in the field and press the right scale button. The left button is to reverse that scale. To increase the size by 10%, enter 110 in the field and press the right scale button.
So, in your example video with 10% in the field, the reverse scale (left) button is the reverse of a 10% scale; if you’d pressed the right scale button with 10%, then the selection would’ve been scaled to 10% of the original and the left button would’ve returned back to original.
Hope that makes sense.
Got it! thanks
Best change you made to G3!