Bengali graphemes can render outside TUI borders

Description

Bengali text in agent or tool output can drift horizontally and render past the right edge of a TUI panel. OpenCode's logical buffer stays aligned, but a terminal that renders multi-code-point Indic graphemes with a wider cell profile advances farther than OpenTUI 0.4.5 expects.

For the sample below, OpenTUI 0.4.5 measures 12 cells. OpenTUI 0.5.9's unicode-wide profile measures 18 cells. The six-cell difference matches the cumulative rightward drift in the affected output.

Plugins

None

OpenCode version

1.18.25

Steps to reproduce

1. Start OpenCode in a terminal that renders multi-code-point Indic graphemes with wide cells.

2. Have an agent or tool print this plain text near the right edge of a bordered output panel:

   বাংলা ভাষায় কথা বলি বাংলা ভাষায় কথা বলি
   

3. Observe the text and the panel's right border.

Expected: the text wraps inside the panel and the border remains aligned.

Actual: Bengali glyphs drift to the right and can render outside the panel.

Operating System

macOS 26.5.2 (Darwin 25.5.0, arm64)

Terminal

Unknown

Additional context

• NFC and NFD are identical for the sample, so normalization is not the cause.
• Plain text follows the same width path, so Markdown and ANSI splitting are not required to reproduce the mismatch.
• OpenCode's current dev branch pins @opentui/core, @opentui/keymap, and @opentui/solid to 0.4.5.
• OpenTUI fixed a related Ghostty wide-grapheme mismatch in anomalyco/opentui#1415, reported as anomalyco/opentui#1401. The fix is included in OpenTUI 0.5.9.
• OpenCode's v2 branch already pins OpenTUI 0.5.9. A safe stable backport or dependency upgrade may therefore be preferable to application-level clipping.

Acceptance criteria

• The Bengali sample remains inside the tool-output panel.
• The right border remains aligned after wrapping, redraw, and terminal resize.
• Existing ASCII and CJK rendering does not regress.