Still in release 1.10.2 but one of the comments solved it for me.
violine1101 added a comment - 09/Apr/15 8:33 PM @Sophia Gardener: To turn off fullscreen on startup, go to your .minecraft folder and open the "options.txt" file. Then change "fullscreen=true" to "fullscreen=false", close Minecraft and save the "options.txt" file. After that, Minecraft shouldn't start in fullscreen mode anymore.
Federico Olivera
Still in release 1.10.2 but one of the comments solved it for me.
violine1101 added a comment - 09/Apr/15 8:33 PM
@Sophia Gardener: To turn off fullscreen on startup, go to your .minecraft folder and open the "options.txt" file. Then change "fullscreen=true" to "fullscreen=false", close Minecraft and save the "options.txt" file. After that, Minecraft shouldn't start in fullscreen mode anymore.
Hope it works for you too, thank you violine1101