Windows 10: Computer switches to internal screen, messing up window sizes and positions
Solution 1:
I have a Sony Vaio laptop with Intel HD Graphics 4000 that I did the same 7 to 10 upgrade with and was experiencing similar display issues. Earlier this week I upgraded to Windows 10 build 10525 and used the Intel driver update utility to update the graphics driver to 10.18.10.4226 (version reported in Device Manager). These were both done on the same day so I can't say which one resolved the problem but it has since gone away.