From aee8555ba782d303d191e1cd2cd3d7c2c1b96ffa Mon Sep 17 00:00:00 2001
From: Rupayon Haldar <80724680+rupayon123@users.noreply.github.com>
Date: Thu, 29 Oct 2026 09:48:38 -0400
Subject: [PATCH] Account for labels in the fast progress bar width

---
 .../performance-and-the-fast-path.rst         |  5 ++
 progressbar/fast.py                           |  4 +-
 tests/test_fast_default.py                    | 50 +++++++++++++++++++
 3 files changed, 58 insertions(+), 1 deletion(-)

diff --git a/docs/explanation/performance-and-the-fast-path.rst b/docs/explanation/performance-and-the-fast-path.rst
index 100e502e..d21b4626 100644
--- a/docs/explanation/performance-and-the-fast-path.rst
+++ b/docs/explanation/performance-and-the-fast-path.rst
@@ -42,6 +42,11 @@ gets the full ``ProgressBar`` instead. You can also construct
 renderer without going through the shortcut -- see
 :doc:`../reference/progressbar`.
 
+The known-length fast bar reserves width for ``prefix`` (or ``desc``)
+and ``suffix`` before sizing its bar. It uses ``custom_len`` to measure
+the fixed text, so ANSI colors and custom display-width rules do not
+consume extra bar columns.
+
 This dispatch only happens inside ``progressbar.progressbar()``.
 Constructing ``ProgressBar()`` yourself always gets the full class.
 
diff --git a/progressbar/fast.py b/progressbar/fast.py
index 3fd71d62..4c737039 100644
--- a/progressbar/fast.py
+++ b/progressbar/fast.py
@@ -76,7 +76,9 @@ def _pure_format_fast_line(bar: FastProgressBar) -> str:
             eta = '--:--:--'
         left = f'{pct:3.0f}% {count} '
         right = f' Elapsed Time: {elapsed_text} ETA: {eta}'
-        inner = max(width - len(left) - len(right) - 2, 0)
+        inner = max(
+            width - bar.custom_len(prefix + left + right + suffix) - 2, 0
+        )
         filled = int(inner * done / total) if total else inner
         barstr = '|' + '#' * filled + ' ' * (inner - filled) + '|'
         return f'{prefix}{left}{barstr}{right}{suffix}'
diff --git a/tests/test_fast_default.py b/tests/test_fast_default.py
index b9fc8700..eaaa92f3 100644
--- a/tests/test_fast_default.py
+++ b/tests/test_fast_default.py
@@ -4,6 +4,8 @@
 import io
 import sys
 
+import pytest
+
 import progressbar
 
 # Alias (not a `from` import) so CodeQL doesn't flag `progressbar` as imported
@@ -248,3 +250,51 @@ def test_fast_path_does_not_import_widgets_or_colors():
     )
     assert out.returncode == 0, out.stderr
     assert 'ok' in out.stdout
+
+
+@pytest.mark.parametrize('width', [80, 110])
+@pytest.mark.parametrize(
+    ('prefix', 'suffix'),
+    [
+        ('', ''),
+        ('load ', ''),
+        ('', ' done'),
+        ('load ', ' done'),
+        ('\x1b[31mload \x1b[0m', ' done'),
+    ],
+)
+def test_fast_labels_share_terminal_width(width, prefix, suffix):
+    fd = TTY()
+    bar = fast_module.FastProgressBar(
+        max_value=10, fd=fd, term_width=width, prefix=prefix, suffix=suffix
+    )
+    assert list(bar(range(10))) == list(range(10))
+    for frame in fd.repaints():
+        assert progressbar.utils.len_color(frame.rstrip('\r\n')) == width
+
+
+def test_fast_labels_use_custom_width_measurement():
+    def display_width(text):
+        return progressbar.utils.len_color(text.replace('界', 'xx'))
+
+    fd = TTY()
+    bar = fast_module.FastProgressBar(
+        max_value=10,
+        fd=fd,
+        term_width=80,
+        prefix='界 ',
+        suffix=' 界',
+        custom_len=display_width,
+    )
+    assert list(bar(range(10))) == list(range(10))
+    for frame in fd.repaints():
+        assert display_width(frame.rstrip('\r\n')) == 80
+
+
+def test_shortcut_desc_shares_terminal_width():
+    fd = TTY()
+    assert list(
+        progressbar.progressbar(range(10), fd=fd, desc='load ', term_width=80)
+    ) == list(range(10))
+    for frame in fd.repaints():
+        assert progressbar.utils.len_color(frame.rstrip('\r\n')) == 80