Fix observeEvent stack trace stripping (#4163) #2669

Job	Run time
routine / routine	4m 6s
website / pkgdown	2m 16s
R-CMD-check / setup	8s
R-CMD-check / macOS-latest (4.4.2)	2m 53s
R-CMD-check / windows-latest (4.4.2)	4m 15s
R-CMD-check / ubuntu-latest (devel)	5m 25s
R-CMD-check / ubuntu-latest (4.4.2)	5m 0s
R-CMD-check / ubuntu-latest (4.3.3)	5m 21s
R-CMD-check / ubuntu-latest (4.2.3)	6m 15s
R-CMD-check / ubuntu-latest (4.1.3)	4m 55s
R-CMD-check / ubuntu-latest (4.0.5)	8m 9s
	48m 43s

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