Cursor drift after auto-wrap when first char of wrapped line is a space

prompt_toolkit Bug Report: Cursor drift after auto-wrap with leading space

Summary

When typing in a TextArea (or BufferControl) with wrap_lines=True and
multiline=False, the cursor visually drifts to the right when a space
character becomes the first character of an auto-wrapped visual line. The
drift equals approximately the number of wraps (e.g., 4 wraps → 4 characters
of drift). Navigating back to the line with arrow keys corrects the cursor
position.

Environment

• prompt_toolkit version: 3.0.x (shipped with Hermes Agent, May 2026)
• Python version: 3.11
• OS: Linux (Ubuntu 24.04)
• Terminal: SSH from Windows (PowerShell, Termius)
• TERM: xterm-256color
• Application: Hermes Agent (hermes --tui)

Steps to Reproduce

1. Open a prompt_toolkit application with a TextArea that has:
   • multiline=False (single line input)
   • wrap_lines=True
   • A prompt (e.g., BeforeInput)
2. Type a long line of text without spaces until it auto-wraps to the next
   visual line
3. Continue typing — the first character typed on the new visual line, if it
   is a space (because the text logically has a space at that point), causes
   the cursor to visually jump ~1 character to the right per wrap
4. The cursor now appears 4+ characters away from where text is actually
   inserted

Additional Observations

• Adding spaces BEFORE the auto-wrap point (pushing the wrap point further
  right) prevents the drift
• With full_screen=False, the drift is ~1 character per wrap (4 wraps = 4
  chars)
• With full_screen=True, the drift becomes a full line offset
• The drift is corrected when the user navigates to the line with arrow keys
  (cursor position is recalculated)

Suspected Root Cause

In BufferControl._create_get_processed_line_func, the translate_rowcol
function computes cursor position as:

Point(x=get_processed_line(row).source_to_display(col), y=row)

The y coordinate is always the buffer row (row), which is 0 for a single
line. For wrapped content, the cursor may be on a different visual row, but
y doesn't account for wrapping. The Window._copy_body renders wrapped
content correctly, but the UIContent.cursor_position uses the unadjusted Y.

Additionally, BeforeInput.apply_transformation uses fragment_list_len
for the source_to_display shift, which counts characters rather than
display width. Multi-width characters (emoji, CJK) are miscounted.

Related Code

• prompt_toolkit/layout/containers.py — Window._copy_body wraps text at
  line 2016+ but doesn't feed wrapping info back to cursor position
• prompt_toolkit/layout/controls.py — BufferControl._create_get_processed_line_func
  line 773: translate_rowcol returns y=row
• prompt_toolkit/layout/processors.py — BeforeInput line 507:
  shift_position = fragment_list_len(fragments_before) should be
  fragment_list_width(fragments_before)

Reproduce the single-line, wrapped TextArea case, then read BufferControl._create_get_processed_line_func in prompt_toolkit/layout/controls.py alongside Window._copy_body in containers.py. Check BeforeInput in processors.py for the display-width behavior, and compare cursor placement for wrapping and multi-width characters. Done means the cursor stays aligned after auto-wrap without breaking existing rendering behavior.