There is something wrong with the re-drawing of the outline view cells, but from the samples you sent, I don’t know what’s causing it.
GarageSale spends it’s time in some system library calls that probably have to do with animations (hence your window server being at 100%), and almost no time in our own code.
Why this is happening in GarageSale 7 and not in GarageSale 8 is beyond me. The mechanism for drawing the outline view is the same, the only difference being that they are linked against different versions of the SDK.
I was really going with the upgrade to Mojave some days ago, then read your message and had some doubts… anyway this morning I will start to work on two machines, the iMac has Mojave and will check if it happens also to me. Until now I worked with transparency ON with Sierra and no problems at all… let’s check later. Anyway this is not the first time I hear about app problems due to transparency…