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Document BottomBar visible window positioning and pixel/DP conversion

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Greg Pomerantz 2026-05-09 17:22:31 -04:00
parent e096341533
commit 3a3a62a50c
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doc/shaper_usage.md
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@ -571,6 +571,24 @@ visualGap = (line2Baseline - line1Baseline) - (ascent1 + ascent2)
With `LineHeightScale = 1.0`: `visualGap ≈ 0` (lines touch)
With `LineHeightScale = 1.2`: `visualGap ≈ 0.2 * lineHeight` (20% padding)
## 9.5 Drawing Elements Inside the Visible Window
When drawing elements like the BottomBar, the region Y computed from `screenHeight` may be outside the visible window area. This happens because `app.ConfigEvent` reports the **physical pixel size** of the window, which can be much larger than the requested DP size (e.g., macOS reports 1560×3376 pixels when we requested 390×844 DP).
**The fix**: Use `gtx.Constraints.Max.Y` (in physical pixels) to clamp the draw position to the visible area:
```go
// gtx.Constraints.Max.Y is in physical pixels.
// Convert to DP using gtx.Metric.PxPerDp.
scale := gtx.Metric.PxPerDp
bottomBarHeightPx := int(24 * scale)
marginPx := int(10 * scale)
drawYPx := gtx.Constraints.Max.Y - bottomBarHeightPx - marginPx
drawY := unit.Dp(float64(drawYPx) / float64(scale))
```
**Key insight**: `gtx.Constraints.Max.X/Y` are in **physical pixels**, not DP. Always convert using `gtx.Metric.PxPerDp` when mixing pixels and DP.
## 10. Summary
- **Always** use `fixed.I(gtx.Sp(size))` for `PxPerEm`
@ -580,6 +598,8 @@ With `LineHeightScale = 1.2`: `visualGap ≈ 0.2 * lineHeight` (20% padding)
- **Baseline spacing** = `lineHeight * LineHeightScale` (default 1.2)
- **For tight multi-line layouts**, use `LineHeightScale: 1.0` or set explicit `LineHeight`
- **Visual gap** between lines = `baselineSpacing - (ascent1 + ascent2)`
- **gtx.Constraints.Max.X/Y** are in **physical pixels**, not DP. Convert using `gtx.Metric.PxPerDp`.
- **When drawing at the bottom of the window**, use `gtx.Constraints.Max.Y - elementHeight - margin` to clamp to visible area.
- **Avoid** using `material.Label()` when you need precise glyph positions
- **Reuse** glyph data for measurement, hit testing, and rendering
- **Follow** Gio's `paintGlyph` as the reference implementation