I am so sick of this issue even though I have a solution but in searching here and everywhere I find nothing but user-based (which should in no way be necessary) work arounds. Ridiculous things like having to move SU back to your main monitor every time before you close the program, an so on.
I run two different external monitors most always (one in one location, and one when Im in another), windows 10, and Sketchup is THE ONLY PROGRAM that EVER opens itself to an unconnected, phantom, external monitor (for instance when you are at a clients house and only on the laptop). This is absolutely, and completely, a Sketchup specific issue. I can have 15 o 20 programs on an external monitor, and another 15 or 20 on the laptop (main monitor) and Sketchup will ALWAYS be the only program that when starting up with no external monitor, will be sitting off in Sketchup la la land.
Anyway, I have seen a post or two about the solution but just to get it on the record for those who get stuck in this SU hell, the solution is to click the program on the task bar until your sure its active (depressed, highlighted, and you’ve see the program sweep off to the left or right and disappear into the Sketchup phantom monitor la la land, and then click ALT+Space and then M and use arrow keys to move it back to the ONLY SCREEN.
Having brought this to their attention countless times, I have no idea why Trimble hasnt addressed this flaw. It makes no sense and is completely annoying and unnecessary to have to deal with.