Experienced roff users expect requests that take numeric values with units to store those quantities in basic units, so that even if font-size-relative units were initially specified, subsequent changes to the font size do not change the magnitude of those stored values.

The .spreadwarn request appears to work in the opposite direction: even if its parameter is specified as an absolute value (e.g., in inches), it seems to be stored as a font-relative one, so that it scales to the current font size.  Examples below illustrate this.

Given what .spreadwarn does, this is a reasonable design, but surprising compared to how most roff requests work, so should be documented.

.spreadwarn is a groffism, though Heirloom troff has also implemented it.

(Not relevant to this issue, but linked here since I chased it down before opening this bug: the .spreadwarn documentation was recently updated in response to bug #58035.)

Examples

$ cat spreadwarn_scaling
.spreadwarn 1
.ll 70p
one two three\p
.ll 75p
one two three\p
.ps 40
.ll 290p
one two three\p
.ll 300p
one two three\p
$ groff -a spreadwarn_scaling
<beginning of page>
one two three
troff: spreadwarn_scaling:5: warning [p 1, 0.2i]: spreading 1.0335m per space
one two three
one two three
troff: spreadwarn_scaling:10: warning [p 1, 0.5i]: spreading 1.0335m per space
one two three

The lack of a warning above about the third output line is mildly surprising, given the way many requests taking a parameter in ems (.spreadwarn's default) store it as an absolute value.  But more surprising (to me) is that this still holds when explicitly providing an invariant unit as the parameter:

$ sed 's/spreadwarn 1/spreadwarn 0.14i/' spreadwarn_scaling | groff -a
<beginning of page>
one two three
troff: <standard input>:5: warning [p 1, 0.2i]: spreading 1.0335m per space
one two three
one two three
troff: <standard input>:10: warning [p 1, 0.5i]: spreading 1.0335m per space
one two three

Both 40-point output lines have well over 0.14 inches of space added, but still only the last gets the warning, as only that one exceeds the threshold in ems.