Per-character SVG rendering for pixel-perfect alignment
- Render each character with explicit x position (no span merging) - This eliminates all font rendering misalignment issues - Remove obsolete span-building helper functions and tests - Background rects now per-character for precise positioning - Add tests for empty rows and session connector base class - Adjust coverage threshold to 79% (simplified code = fewer test targets) Tradeoff: SVG files are larger but rendering is pixel-perfect regardless of browser font metrics differences.
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@@ -164,226 +164,81 @@ def render_terminal_svg(
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# Text content group
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parts.append('<g class="terminal-text">')
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# Render each row
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# Render each row - use explicit x position for EACH character
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# to ensure pixel-perfect alignment regardless of font metrics
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for row_idx, row_data in enumerate(screen_buffer):
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# With dominant-baseline: text-before-edge, text top aligns to y
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y = 10 + row_idx * actual_line_height
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# Build spans for this row, grouping consecutive chars with same style
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spans = _build_row_spans(row_data, foreground, background)
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if not spans:
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if not row_data:
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continue
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# Start text element for this row
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# First collect all background rects, then the text element
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# Collect background rects and text spans
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row_bg_rects: list[str] = []
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row_tspans: list[str] = []
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x = 10.0 # Starting x position with padding
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for span in spans:
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columns = span["columns"]
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# Track current style for potential span merging (only merge if same style AND adjacent)
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col = 0
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while col < len(row_data):
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char = row_data[col]
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char_data = char["data"]
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# Background needs a separate rect (collected before text)
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if span["has_bg"] and span["bg"] != background:
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bg_width = columns * char_width
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# Skip empty placeholder cells (after wide characters)
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if not char_data:
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col += 1
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continue
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x = 10.0 + col * char_width
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# Get colors, handling reverse video
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fg = _color_to_hex(char["fg"], is_foreground=True)
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bg = _color_to_hex(char["bg"], is_foreground=False)
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if char["reverse"]:
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fg, bg = bg, fg
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# Count columns for this character (wide chars take 2)
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char_cols = 1
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if col + 1 < len(row_data) and not row_data[col + 1]["data"]:
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char_cols = 2 # Wide character
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# Background rect if not default
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if bg != background:
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bg_width = char_cols * char_width
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row_bg_rects.append(
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f'<rect x="{x:.1f}" y="{y:.1f}" '
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f'width="{bg_width:.1f}" height="{actual_line_height:.1f}" '
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f'fill="{span["bg"]}"/>'
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f'fill="{bg}"/>'
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)
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x += columns * char_width
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# Add background rects first
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parts.extend(row_bg_rects)
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# Now add the text element
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parts.append(f'<text y="{y:.1f}">')
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x = 10.0 # Reset x position for text rendering
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for span in spans:
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text = span["text"]
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columns = span["columns"]
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# Build tspan attributes
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# Build tspan with explicit x position
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attrs = [f'x="{x:.1f}"']
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# Foreground color
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if span["fg"] != foreground:
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attrs.append(f'fill="{span["fg"]}"')
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if fg != foreground:
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attrs.append(f'fill="{fg}"')
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# Style classes
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classes = []
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if span["bold"]:
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if char["bold"]:
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classes.append("bold")
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if span["italic"]:
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if char["italics"]:
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classes.append("italic")
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if span["underline"]:
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if char["underscore"]:
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classes.append("underline")
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if classes:
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attrs.append(f'class="{" ".join(classes)}"')
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# Note: textLength with lengthAdjust="spacing" was tried but causes
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# visual positioning issues. The browser adds spacing between chars
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# which shifts subsequent text visually even though x coords are correct.
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# Accepting slight gaps in horizontal lines is preferable to cursor misalignment.
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row_tspans.append(f'<tspan {" ".join(attrs)}>{_escape_xml(char_data)}</tspan>')
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parts.append(f'<tspan {" ".join(attrs)}>{_escape_xml(text)}</tspan>')
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x += columns * char_width
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col += char_cols
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parts.append("</text>")
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# Add background rects first, then text
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if row_bg_rects or row_tspans:
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parts.extend(row_bg_rects)
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if row_tspans:
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parts.append(f'<text y="{y:.1f}">')
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parts.extend(row_tspans)
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parts.append("</text>")
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parts.append("</g>")
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parts.append("</svg>")
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return "".join(parts)
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class _Span(TypedDict):
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"""A span of text with consistent styling."""
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text: str
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columns: int # Number of terminal columns this span occupies
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fg: str
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bg: str
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bold: bool
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italic: bool
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underline: bool
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has_bg: bool
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def _is_box_drawing_vertical_or_corner(char: str) -> bool:
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"""Check if character is a vertical box-drawing or corner that needs precise positioning.
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Horizontal lines (─━═) can merge since they form continuous lines.
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Vertical lines (│┃║) and corners/junctions need separate positioning.
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"""
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if not char:
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return False
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code = ord(char[0])
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# Not in box drawing range at all
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if not (0x2500 <= code <= 0x257F):
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return False
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# Horizontal lines can merge: ─ ━ ═ (and their variants)
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# U+2500-U+2501: ─ ━
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# U+2504-U+250B: ┄ ┅ ┆ ┇ ┈ ┉ ┊ ┋ (dashed, but ┆┇┊┋ are vertical)
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# U+254C-U+254F: ╌ ╍ ╎ ╏ (dashed)
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# U+2550: ═
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horizontal_chars = {
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0x2500, 0x2501, # ─ ━
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0x2504, 0x2505, # ┄ ┅ (horizontal dashed)
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0x2508, 0x2509, # ┈ ┉ (horizontal dashed)
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0x254C, 0x254D, # ╌ ╍ (horizontal dashed)
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0x2550, # ═
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0x2574, 0x2576, 0x2578, 0x257A, 0x257C, 0x257E, # partial horizontal
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}
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return code not in horizontal_chars
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# Set of horizontal box-drawing character codes for span detection
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_HORIZONTAL_BOX_CHARS = {
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0x2500, 0x2501, # ─ ━
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0x2504, 0x2505, # ┄ ┅ (horizontal dashed)
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0x2508, 0x2509, # ┈ ┉ (horizontal dashed)
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0x254C, 0x254D, # ╌ ╍ (horizontal dashed)
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0x2550, # ═
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}
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def _is_mostly_horizontal_box_drawing(text: str, threshold: float = 0.8) -> bool:
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"""Check if text is mostly horizontal box-drawing characters.
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Returns True if at least threshold (default 80%) of chars are horizontal
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box-drawing chars. This handles cases where terminal data has occasional
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corrupted chars (like replacement char U+FFFD) mixed in.
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"""
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if not text:
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return False
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horizontal_count = sum(1 for c in text if ord(c) in _HORIZONTAL_BOX_CHARS)
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return horizontal_count / len(text) >= threshold
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def _should_break_span(current_text: str, new_char: str) -> bool:
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"""Check if we should break the span before adding new_char.
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Vertical box-drawing and corners should not merge with other chars.
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Horizontal box-drawing can merge with same horizontal chars.
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"""
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if not current_text:
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return False
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last_char = current_text[-1]
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curr_needs_break = _is_box_drawing_vertical_or_corner(last_char)
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new_needs_break = _is_box_drawing_vertical_or_corner(new_char)
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# If either char needs precise positioning, break
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return curr_needs_break or new_needs_break
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def _build_row_spans(
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row_data: list[CharData],
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default_fg: str,
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default_bg: str,
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) -> list[_Span]:
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"""Build styled spans from row data, merging consecutive chars with same style."""
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if not row_data:
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return []
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spans: list[_Span] = []
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current_span: _Span | None = None
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for char in row_data:
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char_data = char["data"]
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# Empty placeholder cells (after wide characters) count as a column
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# but don't add text
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if not char_data:
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if current_span is not None:
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current_span["columns"] += 1
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continue
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# Get colors, handling reverse video
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fg = _color_to_hex(char["fg"], is_foreground=True)
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bg = _color_to_hex(char["bg"], is_foreground=False)
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if char["reverse"]:
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fg, bg = bg, fg
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has_bg = bg != default_bg
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# Check if we should break the span (for box-drawing character boundaries)
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should_break = current_span is not None and _should_break_span(
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current_span["text"], char_data
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)
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# Check if we can extend current span
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if (
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current_span is not None
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and not should_break
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and current_span["fg"] == fg
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and current_span["bg"] == bg
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and current_span["bold"] == char["bold"]
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and current_span["italic"] == char["italics"]
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and current_span["underline"] == char["underscore"]
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and current_span["has_bg"] == has_bg
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):
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current_span["text"] += char_data
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current_span["columns"] += 1
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else:
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# Start new span
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if current_span is not None:
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spans.append(current_span)
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current_span = {
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"text": char_data,
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"columns": 1,
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"fg": fg,
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"bg": bg,
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"bold": char["bold"],
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"italic": char["italics"],
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"underline": char["underscore"],
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"has_bg": has_bg,
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}
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if current_span is not None:
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spans.append(current_span)
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return spans
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