bug#36733: 27.0.50; Eshell taking long time to enter directory after TAB completion

[Ivan Kozlov]

I have identified the problem. completion-in-region uses markers to identify 
the beginning and the end of the region where the completion is active. The 
markers are saved into the list completion-in-region--data.

completion-in-region--post checks if the point has left the buffer, in which 
case it quits the completion mode (as I have already noted):

                (>= (point) (nth 0 completion-in-region--data))
                (<= (point)
                    (save-excursion
                      (goto-char (nth 1 completion-in-region--data))
                      (line-end-position)))

Erasing text or entering line feed moves markers, it appears to be their whole 
purpose. Here is the problem. The markers move and the conditions above give 
true, so 1) completion-in-mode--predicate is called 2) completion-in-region 
mode remains active, both of which are wrong.

My investigation must end here. I don’t know why completion-in-region-mode is 
using markers (to enter text outside the region you probably need to move the 
point out of it, which exits the mode), and I don’t know if it is possible to 
avoid this situation when using them. All I know is that the current behavior 
is wrong and doesn’t make any sense. I do hope somebody chimes in who has any 
ideas.