This change was reverted in 76ef52267cf887e3e1aa6d25b3b16dd0601dd459.
It also doesn't seem correct. cursor-sensor--detect is only used in pre-redisplay-functions, and the documentation of that variable says:
"Hook run just before redisplay.
It is called in each window that is to be redisplayed. It takes one argument,
which is the window that will be redisplayed. When run, the ‘current-buffer’
is set to the buffer displayed in that window."
That means that (bobp) is correct and (= point 1) cannot give a different result, unless narrowing is in effect (then only bobp is correct).
Given that replacing (bobp) with (= point 1) does solve this bug, the documentation of pre-redisplay-functions must be incorrect, i.e. the current buffer is not the buffer of the window passed as argument. I think the only way how this can happen is that a previous entry in pre-redisplay-functions has changed the current buffer. Probably the implementation of redisplay--pre-redisplay-functions should be changed from
(with-current-buffer (window-buffer win)
(run-hook-with-args 'pre-redisplay-functions win))
(lambda (func) (with-current-buffer (window-buffer win)
(funcall func win)