[bug #64602] [troff] `fl` request behavior difficult to rationalize

[G. Branden Robinson]

URL:
  <https://savannah.gnu.org/bugs/?64602>

                 Summary: [troff] `fl` request behavior difficult to
rationalize
                   Group: GNU roff
               Submitter: gbranden
               Submitted: Sun 27 Aug 2023 08:05:01 PM UTC
                Category: Core
                Severity: 3 - Normal
              Item Group: Incorrect behaviour
                  Status: Need Info
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
         Planned Release: None


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Follow-up Comments:


-------------------------------------------------------
Date: Sun 27 Aug 2023 08:05:01 PM UTC By: G. Branden Robinson <gbranden>
In bug #64484, Deri brought something puzzling to my attention.

Observe the behavior of this input.


$ cat EXPERIMENTS/output-device.groff
foobar
.br
.ds a lorem\"
\X'1 X \(ti \*a'
.device 2 device \(ti \*a
.fl
\!x X 3 ! \(ti \*a
.output x X 4 output \(ti \*a
$ groff -Z EXPERIMENTS/output-device.groff 
x T ps
x res 72000 1 1
x init
p1
x font 5 TR
f5
s10000
V12000
H72000
md
DFd
tfoobar
n12000 0
V24000
H72000
V24000
H72000
x X 1 X ~ lorem
wh2500
V24000
H74500
x X 2 device \(ti lorem
n12000 0
x X 3 ! \(ti lorem
x X 4 output \(ti lorem
x trailer
V792000
x stop


If we remove the `fl` (flush) request, "3" and "4" magically move to precede
"1" and "2".


$ diff -u EXPERIMENTS/output-device.groff EXPERIMENTS/output-device2.groff
--- EXPERIMENTS/output-device.groff     2023-08-27 15:00:33.900430630 -0500
+++ EXPERIMENTS/output-device2.groff    2023-08-27 15:03:43.099885671 -0500
@@ -3,6 +3,5 @@
 .ds a lorem\"
 \X'1 X \(ti \*a'
 .device 2 device \(ti \*a
-.fl
 \!x X 3 ! \(ti \*a
 .output x X 4 output \(ti \*a
$ groff -Z EXPERIMENTS/output-device2.groff 
x T ps
x res 72000 1 1
x init
p1
x font 5 TR
f5
s10000
V12000
H72000
md
DFd
tfoobar
n12000 0
x X 3 ! \(ti lorem
x X 4 output \(ti lorem
V24000
H72000
V24000
H72000
x X 1 X ~ lorem
wh2500
V24000
H74500
x X 2 device \(ti lorem
n12000 0
x trailer
V792000
x stop


What gives?  What's happening, and how do we explain this?







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