Add explicit header dropdown layout types

ui_header_dropdown.go

@@ -0,0 +1,73 @@
+package main
+
+import (
+	"image"
+
+	"gioui.org/layout"
+	"gioui.org/op"
+	"gioui.org/unit"
+)
+
+type dropdownAnchor struct {
+	TriggerRightX  int
+	TriggerBottomY int
+}
+
+func (a dropdownAnchor) point() image.Point {
+	return image.Pt(a.TriggerRightX, a.TriggerBottomY)
+}
+
+type dropdownSurface struct {
+	ContainerWidth int
+	LeftInset      int
+	TopInset       int
+}
+
+func (s dropdownSurface) menuConstraints(gtx layout.Context) layout.Context {
+	menuGTX := gtx
+	menuGTX.Constraints.Min = image.Point{}
+	menuGTX.Constraints.Max.X = max(0, s.ContainerWidth)
+	return menuGTX
+}
+
+func (s dropdownSurface) origin(anchor dropdownAnchor, menuWidth int) image.Point {
+	x := s.LeftInset + anchoredMenuOriginX(s.ContainerWidth, 0, anchor.TriggerRightX, menuWidth)
+	y := s.TopInset + anchor.TriggerBottomY
+	return image.Pt(x, y)
+}
+
+func (s dropdownSurface) draw(gtx layout.Context, anchor dropdownAnchor, menu layout.Widget) layout.Dimensions {
+	menuGTX := s.menuConstraints(gtx)
+	menuOps := op.Record(gtx.Ops)
+	menuDims := layout.Inset{Top: unit.Dp(6)}.Layout(menuGTX, menu)
+	menuCall := menuOps.Stop()
+	menuOrigin := s.origin(anchor, menuDims.Size.X)
+	stack := op.Offset(menuOrigin).Push(gtx.Ops)
+	menuCall.Add(gtx.Ops)
+	stack.Pop()
+	return layout.Dimensions{Size: gtx.Constraints.Max}
+}
+
+type headerActionMetrics struct {
+	RowOriginX int
+	Spacing    int
+	RowDims    layout.Dimensions
+	SyncDims   layout.Dimensions
+	LockDims   layout.Dimensions
+	MainDims   layout.Dimensions
+}
+
+func (m headerActionMetrics) syncAnchor() dropdownAnchor {
+	return dropdownAnchor{
+		TriggerRightX:  m.RowOriginX + m.SyncDims.Size.X,
+		TriggerBottomY: m.RowDims.Size.Y,
+	}
+}
+
+func (m headerActionMetrics) mainAnchor() dropdownAnchor {
+	triggerRightX := m.SyncDims.Size.X + m.Spacing + m.LockDims.Size.X + m.Spacing + m.MainDims.Size.X
+	return dropdownAnchor{
+		TriggerRightX:  m.RowOriginX + triggerRightX,
+		TriggerBottomY: m.RowDims.Size.Y,
+	}
+}