Fix text layout for `align="center", wrap="clip"` when `maxcol` == `line_width - 1` (#543)

* Fix text layout for `align="center", wrap="clip"`

- Fix: Prevent zero run length in text layout.

Refs: urwid#542

* Add test for "clip" text layout fix

Refs: #542

2 files changed, 19 insertions, 1 deletions

diff --git a/urwid/tests/test_text_layout.py b/urwid/tests/test_text_layout.py
index 77bd27d..c87d12c 100644
--- a/urwid/tests/test_text_layout.py
+++ b/urwid/tests/test_text_layout.py
@@ -292,6 +292,23 @@ class CalcTranslateClipTest(CalcTranslateTest, unittest.TestCase):
         [(7, None), (0, 35)],
         [(14, 36, 50), (0, 50)]]
 
+class CalcTranslateClipTest2(CalcTranslateTest, unittest.TestCase):
+    text = "Hello!\nto\nWorld!"
+    mode = 'clip'
+    width = 5  # line width (of first and last lines) minus one
+    result_left = [
+        [(6, 0, 6), (0, 6)],
+        [(2, 7, 9), (0, 9)],
+        [(6, 10, 16), (0, 16)]]
+    result_right = [
+        [(-1, None), (6, 0, 6), (0, 6)],
+        [(3, None), (2, 7, 9), (0, 9)],
+        [(-1, None), (6, 10, 16), (0, 16)]]
+    result_center = [
+        [(6, 0, 6), (0, 6)],
+        [(2, None), (2, 7, 9), (0, 9)],
+        [(6, 10, 16), (0, 16)]]
+
 class CalcTranslateCantDisplayTest(CalcTranslateTest, unittest.TestCase):
     text = b'Hello\xe9\xa2\x96'
     mode = 'space'
diff --git a/urwid/text_layout.py b/urwid/text_layout.py
index aacb36d..8813cd9 100644
--- a/urwid/text_layout.py
+++ b/urwid/text_layout.py
@@ -139,7 +139,8 @@ class StandardTextLayout(TextLayout):
                 out.append([(width-sc, None)] + l)
                 continue
             assert align == 'center'
-            out.append([((width-sc+1) // 2, None)] + l)
+            pad_trim_left = (width-sc+1) // 2
+            out.append([(pad_trim_left, None)] + l if pad_trim_left else l)
         return out
 
     def calculate_text_segments(