Since updating to Minecraft 1.12.2, the game crashes whenever alt-tabbing out of the main screen when full-screened. It does, however, seem to be able to handle alt-tabbing out from the menu in-game.
Output from the Console log:
14:30:46 launcher Preparing to launch minecraft client for 1.12.2
14:30:46 launcher Checking installations.
14:30:46 launcher Minecraft client 1.12.2 is ready to start.
14:30:46 launcher Starting!
14:30:47 launcher Using default game log configuration client-1.12.xml (outputs XML)
14:30:48 bib Setting user: Ommitted for Privacy
14:30:50 bid Skipping bad option: lastServer:
14:30:50 bib LWJGL Version: 2.9.4
14:30:51 cev Reloading ResourceManager: Default, PaintingMod
14:30:52 chm Starting up SoundSystem...
14:30:52 chm Initializing LWJGL OpenAL
14:30:52 chm (The LWJGL binding of OpenAL. For more information, see http://www.lwjgl.org)
14:30:52 chm OpenAL initialized.
14:30:52 chm Sound engine started
14:30:57 cdp Created: 1024x512 textures-atlas
14:30:59 com.mojang.text2speech.NarratorWindows Narrator library for x64 successfully loaded
Attached: One screen shot showing what Minecraft looks like after I have alt-tabed away and back. It looks like the title screen shrunk down to a tiny corner, with the rest of the screen white. Also shown: Windows standard "unresponsive application" dialogue box.
Second screenshot is showing the output of the log and the launcher's "Minecraft crashed" message. The console does not list anything that looks like a crash explanation.