fix(textfield): cursor position drifts on wide Cyrillic glyphs (ы, ш, щ, ж)

[kolkov]

Bug

Cursor position in TextField drifts rightward when typing wide Cyrillic characters (ы, ш, щ, ж). Narrow Latin characters track correctly. After 10+ wide characters the drift reaches 5-10px.

Reproduce

1. go run ./examples/gallery/
2. Click TextField
3. Type ыыыыыыыыыыыыыыыыыы (hold ы key)
4. Cursor visually drifts ahead of the actual insertion point

Root Cause

Two separate bugs in two repos:

1. Base offset rounding mismatch (ui)

DrawText rounds the starting X position: math.Round(contentRect.Min.X) (internal/render/canvas.go:488).
CursorX uses raw contentRect.Min.X without rounding (internal/textmetrics/textmetrics.go:35).

Creates a constant cursor-to-text offset of up to 0.5px.

2. Per-glyph hinted/raw advance mismatch (gg#479)

MeasureText → face.Advance() returns raw unhinted per-glyph advances (text/face.go:88).
DrawText → drawGlyphs → hintedOrRawAdvance() returns TT-hinted advances (text/draw.go:136).

Hinting adjusts wide Cyrillic characters (ш: 3 vertical stems, щ: 4, ж: 5) to pixel grid. Each glyph 0.5-1px wider/narrower than raw advance. After 10 characters → 5-10px cumulative drift.

GPU GlyphMask path is NOT affected — both measurement and rendering use raw advances.

Enterprise reference

Flutter (TextPainter + HarfBuzz), Qt (QFontMetrics + QTextEngine), Chrome (Font::width + ShapeResult) — all use a single shaped result for both rendering and cursor positioning.

Fix

• ui: Round base X in CursorX and RuneIndexFromX to match DrawText. ~20 LOC.
• gg: fix(text): MeasureText returns raw unhinted advances, drawGlyphs uses hinted — cursor drift on wide glyphs gg#479 — Advance() must return hinted advances when hinting is active. ~40-60 LOC.

Environment

• Windows 10, Vulkan backend
• v0.1.50+ (main, post SceneCache refactoring)
• Inter font (has TT bytecode hints)
• Default hinting: HintingFull

[kolkov]

Fixed in PR #213 (cursor rounding + hinted advances via gg v0.50.13)