ui: do not write() to channel if builtin pager has terminated

CHANGELOG.md

@@ -117,6 +117,8 @@ to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 * Windows binaries no longer require `vcruntime140.dll` to be installed
   (normally through Visual Studio.)
 
+* On quit, the builtin pager no longer waits for all outputs to be discarded.
+
 * `jj branch rename` no longer shows a warning in colocated repos.
 
 ## [0.19.0] - 2024-07-03

cli/src/ui.rs

@@ -58,6 +58,11 @@ impl Write for &BuiltinPager {
     }
 
     fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        // Since minus::Pager holds the rx end internally, push_str() doesn't
+        // fail even after the paging thread gets terminated.
+        if self.dynamic_pager_thread.is_finished() {
+            return Err(io::ErrorKind::BrokenPipe.into());
+        }
         let string = std::str::from_utf8(buf).map_err(io::Error::other)?;
         self.pager.push_str(string).map_err(io::Error::other)?;
         Ok(buf.len())