debugui: improve the slider behavior to enable to select the maximum value

Author: hajimehoshi

example/gallery/main.go

@@ -47,6 +47,7 @@ type Game struct {
 	num3_1       float64
 	num3_2       float64
 	num4         float64
+	num5         int
 }
 
 func NewGame() (*Game, error) {

example/gallery/ui.go

@@ -156,6 +156,7 @@ func (g *Game) testWindow(ctx *debugui.Context) {
 			ctx.NumberFieldF(&g.num3_1, 0.1, 2)
 			ctx.NumberFieldF(&g.num3_2, 0.1, 2)
 			ctx.SliderF(&g.num4, 0, 10, 0.1, 2)
+			ctx.Slider(&g.num5, 0, 2, 1)
 		})
 		ctx.Header("Licenses", false, func() {
 			ctx.Text(`The photograph by Chris Nokleberg is licensed under the Creative Commons Attribution 4.0 License

slider.go

@@ -69,7 +69,7 @@ func (c *Context) slider(value *int, low, high, step int, id widgetID, opt optio
 		var e EventHandler
 		if c.focus == id && c.pointing.pressed() {
 			if w := bounds.Dx() - defaultStyle.thumbSize; w > 0 {
-				v = low + (c.pointingPosition().X-bounds.Min.X)*(high-low)/w
+				v = low + (c.pointingPosition().X-bounds.Min.X-defaultStyle.thumbSize/2)*(high-low+step)/w
 			}
 			if step != 0 {
 				v = v / step * step
@@ -112,7 +112,7 @@ func (c *Context) sliderF(value *float64, low, high, step float64, digits int, i
 		var e EventHandler
 		if c.focus == id && c.pointing.pressed() {
 			if w := float64(bounds.Dx() - defaultStyle.thumbSize); w > 0 {
-				v = low + float64(c.pointingPosition().X-bounds.Min.X)*(high-low)/w
+				v = low + float64(c.pointingPosition().X-bounds.Min.X-defaultStyle.thumbSize/2)*(high-low+step)/w
 			}
 			if step != 0 {
 				v = math.Round(v/step) * step