thanks for the feedback, snowisheres and sillytweet5, but i know how creepers work. i've been playing this game since v1.6.2. i'm saying the fact that a creeper can kill you during the day is a bug. period.
found a workaround - manually edit ~/.minecraft/window_settings_launcher.conf
I can confirm the issue is resolved with launcher 1.6.89-j.
Confirmed, Minecraft 1.12.2, Java 1.8.0_151, Debian 9.2 (Xfce).
My current workaround is to manually edit options.txt (overrideWidth:1600, overrideHeight:900), which works until I switch to fullscreen and back while in game. This resets the resolution back to the default 854 x 480. I have to quit and reload to bring the resolution back to 1600 x 900.
welp, i guess my minecraft playing days have come to an end. i haven't been able to win in hardcore since v1.14.4 and the game has been too op since the nether update. day creepers and the uselessness of unenchanted armour (two arrow kill by skeletons, two hit kill by baby zombies, one hit kill by endermen with full iron armour? come on!), as well as other annoying "intentional" aspects of the game have finally broken me. i get that hardcore should be challenging, but the game should also be fun. i no longer have fun playing. that day creeper ruined nearly a decade of enjoyment for me 😞