Widget display width

[rorygraves]

DECISIONS — widget-display-width (#206)

Follow-up to #205. Converted the last two widgets that still sized text by UTF-16
String.length to measure by display cells (WCWidth).

Changes

MenuBar.scala

• dropdownInnerWidth: _.length → WCWidth.stringWidth (widest item by cells).
• dropdown rows: item.padTo(innerWidth, ' ') → fitToCells(item, innerWidth)
  (cell-aware clip + pad; ASCII unchanged).
• titleStartCol / hitTestTitles: title.length + 4 → WCWidth.stringWidth(title) + 4.
  The +4 (2-cell opener + 2-cell closer, same for [ X ] and X ) is unchanged;
  only the title measurement moved to cells, so a wide-char title no longer
  mis-positions later dropdowns or mislands title hit-testing.

ListView.scala

• view: content.take(width-2).padTo(width-2, ' ') → fitToCells(content, width-2).
  Old .take could slice a surrogate pair / wide glyph at the boundary and could
  overflow the row (UTF-16 count < display cells for wide content).

Key decision: helpers duplicated, not centralised

The #205 clipToCells / fitToCells are private to Table, and #205 itself
already duplicated the same logic privately into TextField and MultiLineInput
(clipToWidth/padToWidth). I followed that established per-widget-private pattern
rather than extracting a shared util — keeps this change minimal and consistent with
the codebase as it stands. The copies in MenuBar/ListView are byte-identical to
Table's. If a future task wants one home for these, WCWidth (termflow-terminal) is
the natural place; that refactor is out of scope here.

Tests

• MenuBarSpec: wide-title (文件) hit-testing positions the next menu by cells;
  dropdown rows with a wide item (打开) pad to display width; open dropdown aligns
  under the correct menu. (Old .length math would have placed Edit at col 6 not 10.)
• ListViewSpec: CJK row aligns to the grid; wide content truncates on a glyph
  boundary (odd content width can't fit a half glyph); emoji (surrogate pairs)
  truncate without leaving a lone surrogate half.
• Rendered-cell assertions reconstruct logical content via RenderCell.renderedGlyph
  (skipping the width-0 filler the renderer inserts after a wide glyph), since the raw
  .ch grid interleaves a space after each 2-cell glyph.
• Full termflowWidgets/test green (309 tests).

Risk

Low. Behaviour identical for pure-ASCII (stringWidth == length, fitToCells ==
take+padTo). WCWidth width tables (ambiguous-width = 1, curated emoji ranges)
are inherited from #205 and unchanged.

[codecov]

Codecov Report

❌ Patch coverage is 93.93939% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...src/main/scala/termflow/tui/widgets/ListView.scala	93.33%	1 Missing ⚠️
.../src/main/scala/termflow/tui/widgets/MenuBar.scala	94.44%	1 Missing ⚠️

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