fix(text area): fix initial flicker in rendering

Fix initial flicker in the `TextArea`, where for a brief moment each
character is rendered on a new line before the size is settled.

The width for the `WrappedDocument` is _slightly_ confusing where zero
means no wrapping, but the current negative wrap width looks like the
bug.

Fixes #5841

Author: TomJGooding

src/textual/widgets/_text_area.py

@@ -1021,7 +1021,7 @@ def wrap_width(self) -> int:
         width, _ = self.scrollable_content_region.size
         cursor_width = 1
         if self.soft_wrap:
-            return width - self.gutter_width - cursor_width
+            return max(0, width - self.gutter_width - cursor_width)
         return 0
 
     def _rewrap_and_refresh_virtual_size(self) -> None: