tui: fix sidebar background color when collapsed #1

Job	Run time
compute-hash (x86_64-linux, blacksmith-4vcpu-ubuntu-2404)	1s
compute-hash (aarch64-linux, blacksmith-4vcpu-ubuntu-2404-arm)	1s
compute-hash (x86_64-darwin, macos-15-intel)	9m 45s
compute-hash (aarch64-darwin, macos-latest)	9m 30s
update-hashes	-1s
	19m 16s

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