progressbar: fix regression with hidden bars

The documentation of the progress bar states that the progress bar
will output only the label of the bar if it is outputting to a file
that is not a tty. However, this was broken a few versions ago,
without the documentation being adjusted, and even a test added.

Restore this behaviour so we follow the documentation again, and
adjust the test to match. Fixes #1138.

1 files changed, 6 insertions, 0 deletions

diff --git a/src/click/_termui_impl.py b/src/click/_termui_impl.py
index d4e3e88..7029a08 100644
--- a/src/click/_termui_impl.py
+++ b/src/click/_termui_impl.py
@@ -227,6 +227,12 @@ class ProgressBar:
         import shutil
 
         if self.is_hidden:
+            # Only output the label as it changes if the output is not a
+            # TTY. Use file=stderr if you expect to be piping stdout.
+            if self._last_line != self.label:
+                self._last_line = self.label
+                echo(self.label, file=self.file, color=self.color)
+
             return
 
         buf = []