fix(gantt): remove gantt splitter double border #3348

Job	Run time
Status check > Unit	1s
CI \| Unit	1s
CI \| Visual	1s
Compile themes / run	37s
Lint scripts / run	1m 0s
Render test pages / verify	7s
Lint styles / run (default)	2m 17s
Lint styles / run (bootstrap)	2m 18s
Lint styles / run (material)	2m 14s
Lint styles / run (classic)	2m 16s
Lint styles / run (nouvelle)	43s
Lint styles / run (fluent)	43s
Render test pages / run	2m 22s
Status check > Lint scripts	1s
Status check > Lint styles	1s
Status check > A11y / prepare	1s
Create screenshots / run (default)	2m 11s
Create screenshots / run (bootstrap)	2m 40s
Create screenshots / run (material)	2m 38s
Create screenshots / run (classic)	2m 33s
Create screenshots / run (nouvelle)	1m 52s
Create screenshots / run (fluent)	2m 24s
Status check > A11y / run-checks (1)	2m 33s
Status check > A11y / run-checks (2)	2m 28s
Status check > A11y / run-checks (3)	2m 24s
Status check > A11y / run-checks (4)	3m 5s
Detect changes	16s
Status check > Visual	1s
	39m 48s

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