Okay, I think i have a solution!
Around line 867 of FleeceWindow.cs, there's an if statement that looks like
"if(currentMousePos != mouseDragStart)..."
change that to...
"if(drawingSelectionBox)"
I think this was some old code I forgot to update. Let me know if this works for you! I was able to repro the bug in a *similar* way initially, but not exactly. But the box doesn't seem to appear for me anymore after this.