Research: Text Selection Broken on Agent Response Panels

Research -- Text Selection Broken on Agent Response Panels

Date: 2026-02-07 Owner: agent Phase: Research git_commit: b025379999e67ebc1eb0be3cc387bdfe9e0212a8 tags: text-selection, copy-to-clipboard, RichVisual, offset-metadata, CopyOnSelectStatic last_updated: 2026-02-07 last_updated_note: Root cause identified -- RichVisual missing offset metadata

Goal

Determine why text selection (highlight-to-copy) doesn't work on agent response panels. Users must hold Shift to select, get no visual feedback, while other widgets in the TUI support free selection with auto-copy.

Root Cause (Confirmed)

RichVisual.render_strips() does not inject offset metadata into rendered segments. Textual's text selection system requires offset metadata in each segment's style to map screen coordinates to text positions. Without it, selection never starts.

The Selection Chain (How It Breaks)

1. User clicks on agent panel text
2. screen.py:1581  -- get_widget_and_offset_at(x, y) called
3. _compositor.py:927-947  -- Iterates rendered segments looking for style._meta["offset"]
4. _compositor.py:949  -- No offset found -> returns (widget, None)
5. screen.py:1591  -- select_offset is None -> _select_start is NEVER SET
6. screen.py:1547  -- On drag, _selecting=True but no _select_start -> no selection range
7. Result: No visual highlight, no copy, selection doesn't happen

Why Other Widgets Work

When CopyOnSelectStatic(Text("plain text")) is created:

visualize() at visual.py:89-90
  -> isinstance(obj, Text) is True
  -> Content.from_rich_text(obj, console)  [converts to Content]

Content.render_strips() at content.py:1478 calls rich_style_with_offset(x, y) on every segment, embedding {"offset": (x, y)} in the style meta. Selection works.

When CopyOnSelectStatic(Group(Markdown(...))) is created (agent responses):

visualize() at visual.py:88,92-93
  -> is_renderable(obj) is True, not isinstance(obj, Text)
  -> RichVisual(widget, rich_cast(obj))  [wraps in RichVisual]

RichVisual.render_strips() at visual.py:291-321 renders through Rich's console but never adds offset metadata. The selection and selection_style parameters are accepted but ignored.

Evidence

Note: This issue existed BEFORE the panel-to-CSS migration. Panel(Group(...)) also goes through RichVisual because Panel is a Rich renderable, not a Text. The migration didn't cause this; it just became more noticeable.

Relevant Files

Proposed Fix

Approach: Create SelectableRichVisual that injects offset metadata

Override RichVisual.render_strips() to post-process strips with offset metadata, enabling Textual's selection system to work with Rich renderables.

Where: src/tunacode/ui/widgets/chat.py

What:

1. Create a SelectableRichVisual(RichVisual) subclass
2. Override render_strips() to:
   • Call parent to get the raw strips
   • Iterate strips adding offset metadata ({"offset": (x, y)}) to each segment's style via Style._meta
   • Apply selection highlighting when selection is not None
3. Override visual property in CopyOnSelectStatic to use SelectableRichVisual instead of letting visualize() return plain RichVisual

Key detail: The offset metadata format is {"offset": (x, y)} where x = character position within the line and y = line number. This must be injected via rich.style.Style(meta={"offset": (x, y)}) or by calling Style.__add__(Style(meta={"offset": (x, y)})) on each segment's existing style.

Alternative: Pre-render to Text

Pre-render the Group(Markdown(...)) using console.render_lines() to get styled text, then flatten to a multi-line Text object. Pass that Text to CopyOnSelectStatic so visualize() routes it through Content.from_rich_text().

Trade-off: Simpler but may lose some Rich rendering fidelity (e.g., inline objects, custom padding in code blocks).

Syntax Highlighting (Not Broken)

Initial investigation confirmed that Rich syntax highlighting in code blocks IS correctly preserved through the _Styled wrapper. The CSS color property acts as a fallback, not an override. Monokai colors survive because Style._add() at rich/style.py:737 gives precedence to the segment's color when present.

This was a red herring -- the actual issue is text selection, not syntax highlighting.

Key Patterns / Solutions Found

• RichVisual is selection-blind: It accepts selection and selection_style params in render_strips() but ignores them completely.
• Offset metadata is the selection key: Without meta={"offset": (x, y)} on segment styles, _compositor.get_widget_and_offset_at() returns (widget, None), preventing selection start.
• Content vs RichVisual split: visualize() at visual.py:88-93 routes Text to Content (selectable) and everything else to RichVisual (not selectable). This is the fundamental branch point.
• Style._add preserves inline colors: CSS base style acts as fallback; Rich inline styles take precedence. Not the issue but important for the fix (offset meta must be added without breaking existing styles).

Knowledge Gaps

1. How Textual's selection highlighting visual is rendered (the screen--selection component style) -- need to apply this in the custom render_strips
2. Whether Selection.extract() works correctly with reconstructed offset metadata from RichVisual output
3. Whether rich_style_with_offset from textual.style.Style can be reused or if we need to build the meta manually on rich.style.Style objects

References

• Textual RichVisual.render_strips: .venv/lib/python3.13/site-packages/textual/visual.py:291-321
• Textual Content.render_strips: .venv/lib/python3.13/site-packages/textual/content.py:1460-1510
• Textual _compositor.get_widget_and_offset_at: .venv/lib/python3.13/site-packages/textual/_compositor.py:889-949
• Textual Screen._forward_event (selection handler): .venv/lib/python3.13/site-packages/textual/screen.py:1578-1599
• Textual visualize(): .venv/lib/python3.13/site-packages/textual/visual.py:65-102
• Rich Style._add: .venv/lib/python3.13/site-packages/rich/style.py:729-751
• Commit 2cd1d750: Panel-to-CSS migration
• Commit 0fd3c798: Auto-copy highlighted text feature