I think Xorg just kind of crashed. While browsing my files in Xfe, the hole display got heavily distorted but could be redrawn by minimizing some windows. Xfe couldn't draw any fonts anymore but Firefox continued to work.
Xorg.log attached containing some error messages.
EDIT:
dmesg might be interesting too:
[drm] stuck on render ring
[drm] GPU HANG: ecode 3:0:0x3cd5f8c1, in Xorg [3228], reason: Ring hung, action: reset
[drm] GPU hangs can indicate a bug anywhere in the entire gfx stack, including userspace.
[drm] Please file a _new_ bug report on bugs.freedesktop.org against DRI -> DRM/Intel
[drm] drm/i915 developers can then reassign to the right component if it's not a kernel issue.
[drm] The gpu crash dump is required to analyze gpu hangs, so please always attach it.
[drm] GPU crash dump saved to /sys/class/drm/card0/error
i915: render error detected, EIR: 0x00000010
i915: page table error
i915: PGTBL_ER: 0x01000002
[drm:0xf8d570c1] *ERROR* EIR stuck: 0x00000010, masking
------------[ cut here ]------------
WARNING: CPU: 0 PID: 18654 at drivers/gpu/drm/i915/intel_display.c:3293 0xf8d6f639()
WARN_ON(ret)
Modules linked in: option usb_wwan usbserial ipt_REJECT nf_reject_ipv4 xt_conntrack xt_LOG nf_conntrack_ftp iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat nf_conntrack cpufreq_conservative cpufreq_stats cpufreq_powersave cpufreq_userspace i915 i2c_i801 snd_hda_codec_realtek snd_hda_codec_generic snd_hda_intel drm_kms_helper snd_hda_codec drm snd_hwdep snd_hda_core snd_pcm snd_timer snd intel_agp intel_gtt soundcore i2c_algo_bit agpgart ums_realtek msi_wmi sparse_keymap wmi video backlight battery ac squashfs lz4_decompress pcspkr lpc_ich acpi_cpufreq r8169 mii loop
CPU: 0 PID: 18654 Comm: kworker/u4:0 Not tainted 4.2.7-tinycore #1999
Hardware name: MICRO-STAR INTERNATIONAL CO., LTD U90/U100/U90/U100, BIOS 4.6.3 12/01/2009
Workqueue: i915-hangcheck 0xf8d57253
00000000 c053bc01 f8dc0d75 c01392a6 f8d6f639 f48b0000 f5e55034 f5e55000
f48b0000 c01392df 00000009 c2c83dec f8dc0d75 c2c83e04 f8d6f639 f8dbe0ff
00000cdd f8dc0d75 01170000 f0170000 f5ea3ca0 f5e55208 f83040f1 00000000
Call Trace:
[<c053bc01>] ? 0xc053bc01
[<c01392a6>] ? 0xc01392a6
[<f8d6f639>] ? 0xf8d6f639
[<c01392df>] ? 0xc01392df
[<f8d6f639>] ? 0xf8d6f639
[<f83040f1>] ? 0xf83040f1
[<f8d5e436>] ? 0xf8d5e436
[<f8d709a8>] ? 0xf8d709a8
[<f8d5e436>] ? 0xf8d5e436
[<f8d70ac3>] ? 0xf8d70ac3
[<f8d571c7>] ? 0xf8d571c7
[<c01658f4>] ? 0xc01658f4
[<c0165a03>] ? 0xc0165a03
[<f8d5757e>] ? 0xf8d5757e
[<c014f042>] ? 0xc014f042
[<c0147348>] ? 0xc0147348
[<c01477d1>] ? 0xc01477d1
[<c0147628>] ? 0xc0147628
[<c014a8fd>] ? 0xc014a8fd
[<c012fbf7>] ? 0xc012fbf7
[<c053fdc1>] ? 0xc053fdc1
[<c014a881>] ? 0xc014a881
---[ end trace 08ea147f973a608c ]---
drm/i915: Resetting chip after gpu hang