When I move a layer I get a blue line with a circle at the left hand end to indicate where the layer will be inserted.
After using Pixelmator for a while I find that I no longer get the indicator when moving a layer.
The layer still moves fine but It can take a bit of guesswork to determine if the layer will be appended to the end of a group or below it.
Closing and relaunching Pixelmator fixes the problem (until it happens again).
If I find a specific cause I'll add to this, but I invite anyone who has seen this and knows what triggers it to post a response.
MacBook Pro (13-inch, Mid 2012)
PixelmatorVersion 3.4.4 Twist (60407)