When you hover over text in chat that displays other text with the showText feature, all of Minecraft's sign lighting darkens slightly.
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[media][media]

MC-51298 says the opposite...
Please force a crash by pressing F3 + C for 10 seconds while in-game and attach the crash report ([minecraft/crash-reports/crash-<DATE>-client.txt|http://hopper.minecraft.net/help/finding-minecraft-data-folder]
) here.

My bad, sorry.