Fix dedent at beginning of line

babi.py

@@ -1052,7 +1052,7 @@ class File:
         n = self._dedent_line(self.lines[self.y])
         if n:
             self.lines[self.y] = self.lines[self.y][n:]
-            self.x = self.x_hint = self.x - n
+            self.x = self.x_hint = max(self.x - n, 0)
 
     def shift_tab(self, margin: Margin) -> None:
         if self.select_start:

tests/conftest.py

@@ -55,6 +55,8 @@ class Screen:
         self.lines[y] = (line[:x] + s + line[x:])[:self.width]
 
     def move(self, y, x):
+        assert 0 <= y < self.height
+        assert 0 <= x < self.width
         print(f'MOVE: y: {y}, x: {x}')
         self.y, self.x = y, x
 

tests/indent_test.py

@@ -86,6 +86,14 @@ def test_dedent_selection(run, tmpdir):
         h.await_text('\n1\n2\n    3\n')
 
 
+def test_dedent_beginning_of_line(run, tmpdir):
+    f = tmpdir.join('f')
+    f.write('    hi\n')
+    with run(str(f)) as h, and_exit(h):
+        h.press('BTab')
+        h.await_text('\nhi\n')
+
+
 def test_dedent_selection_does_not_make_selection_negative(run):
     with run() as h, and_exit(h):
         h.press('Tab')