From d435f5a6a977f1c41a228d0a9a824bed3cfd6802 Mon Sep 17 00:00:00 2001
From: Sasha Koshka <sashakoshka@tebibyte.media>
Date: Sat, 21 Jan 2023 01:15:30 -0500
Subject: [PATCH] Scrollbars can be dragged with the mouse

---
 elements/basic/scrollcontainer.go | 51 +++++++++++++++++++++++++++++--
 1 file changed, 48 insertions(+), 3 deletions(-)

diff --git a/elements/basic/scrollcontainer.go b/elements/basic/scrollcontainer.go
index ed36572..a5a372a 100644
--- a/elements/basic/scrollcontainer.go
+++ b/elements/basic/scrollcontainer.go
@@ -19,6 +19,8 @@ type ScrollContainer struct {
 	horizontal struct {
 		exists bool
 		enabled bool
+		dragging bool
+		dragOffset image.Point
 		gutter image.Rectangle
 		bar image.Rectangle
 	}
@@ -26,6 +28,8 @@ type ScrollContainer struct {
 	vertical struct {
 		exists bool
 		enabled bool
+		dragging bool
+		dragOffset image.Point
 		gutter image.Rectangle
 		bar image.Rectangle
 	}
@@ -106,19 +110,40 @@ func (element *ScrollContainer) HandleKeyUp (key tomo.Key, modifiers tomo.Modifi
 }
 
 func (element *ScrollContainer) HandleMouseDown (x, y int, button tomo.Button) {
-	if child, ok := element.child.(tomo.MouseTarget); ok {
+	point := image.Pt(x, y)
+	if point.In(element.horizontal.gutter) {
+		element.horizontal.dragging = true
+		element.dragHorizontalBar(point)
+		
+	} else if point.In(element.vertical.gutter) {
+		element.vertical.dragging = true
+		element.dragVerticalBar(point)
+		
+	} else if child, ok := element.child.(tomo.MouseTarget); ok {
 		child.HandleMouseDown(x, y, button)
 	}
 }
 
 func (element *ScrollContainer) HandleMouseUp (x, y int, button tomo.Button) {
-	if child, ok := element.child.(tomo.MouseTarget); ok {
+	if element.horizontal.dragging {
+		element.horizontal.dragging = false
+		
+	} else if element.vertical.dragging {
+		element.vertical.dragging = false
+		
+	} else if child, ok := element.child.(tomo.MouseTarget); ok {
 		child.HandleMouseUp(x, y, button)
 	}
 }
 
 func (element *ScrollContainer) HandleMouseMove (x, y int) {
-	if child, ok := element.child.(tomo.MouseTarget); ok {
+	if element.horizontal.dragging {
+		element.dragHorizontalBar(image.Pt(x, y))
+		
+	} else if element.vertical.dragging {
+		element.dragVerticalBar(image.Pt(x, y))
+		
+	} else if child, ok := element.child.(tomo.MouseTarget); ok {
 		child.HandleMouseMove(x, y)
 	}
 }
@@ -324,6 +349,26 @@ func (element *ScrollContainer) drawVerticalBar () {
 		element.vertical.bar)
 }
 
+func (element *ScrollContainer) dragHorizontalBar (mousePosition image.Point) {
+	scrollX :=
+		float64(element.child.ScrollContentBounds().Dx()) /
+		float64(element.horizontal.gutter.Dx()) *
+		float64 (
+			mousePosition.X - element.horizontal.bar.Dx() / 2)
+	scrollY := element.child.ScrollViewportBounds().Min.Y
+	element.child.ScrollTo(image.Pt(int(scrollX), scrollY))
+}
+
+func (element *ScrollContainer) dragVerticalBar (mousePosition image.Point) {
+	scrollY :=
+		float64(element.child.ScrollContentBounds().Dy()) /
+		float64(element.vertical.gutter.Dy()) *
+		float64 (
+			mousePosition.Y - element.vertical.bar.Dy() / 2)
+	scrollX := element.child.ScrollViewportBounds().Min.X
+	element.child.ScrollTo(image.Pt(scrollX, int(scrollY)))
+}
+
 func (element *ScrollContainer) updateMinimumSize () {
 	width  := theme.Padding() * 2
 	height := theme.Padding() * 2