Man... the graphical glitches have returned! Same symptoms as last time, but this time, I noticed a suspicious-looking message in dmesg (which, by the way, is being spammed with "atkbd.c: Unknown key pressed (translated set 2, code 0x8d on isa0060/serio0)."; I need a new laptop).
[ 17.692023] [drm] radeon kernel modesetting enabled.
[ 17.697903] radeon 0000:01:05.0: power state changed by ACPI to D0
[ 17.697918] radeon 0000:01:05.0: PCI INT A -> GSI 17 (level, low) -> IRQ 17
[ 17.708211] [drm] radeon: Initializing kernel modesetting.
[ 17.718146] [drm] register mmio base: 0xC0100000
[ 17.718151] [drm] register mmio size: 65536
[ 17.718459] [drm] GPU reset succeed (RBBM_STATUS=0x00000140)
[ 17.718486] [drm:rs400_gart_adjust_size] *ERROR* Forcing to 32M GART size (because of ASIC bug ?)
[ 17.718532] [drm] Generation 2 PCI interface, using max accessible memory
[ 17.718535] [drm] radeon: VRAM 128M
[ 17.718537] [drm] radeon: VRAM from 0x38000000 to 0x3FFFFFFF
[ 17.718540] [drm] radeon: GTT 32M
[ 17.718542] [drm] radeon: GTT from 0x40000000 to 0x41FFFFFF
[ 17.718580] [drm] radeon: irq initialized.