Make scrollbar clickable and draggable

internal/action/actions.go

@@ -57,6 +57,32 @@ func (h *BufPane) ScrollReachedEnd() bool {
 // MousePress is the event that should happen when a normal click happens
 // This is almost always bound to left click
 func (h *BufPane) MousePress(e *tcell.EventMouse) bool {
+	// Scrollbar hit-test
+	// Intercept clicks on the scrollbar before any buffer/cursor handling.
+	if bw, ok := h.BWindow.(*display.BufWindow); ok {
+		mx, my := e.Position()
+		sbX := bw.ScrollBarX()
+		if sbX >= 0 && mx == sbX {
+			trackStart, trackEnd := bw.ScrollBarTrackBounds()
+			if my >= trackStart && my <= trackEnd {
+				thumbStart, thumbEnd := bw.ScrollBarBounds()
+				if my >= thumbStart && my <= thumbEnd {
+					// Click lands on the thumb: start a drag.
+					bw.BeginScrollDrag(my, thumbStart)
+				} else {
+					// Click on the track outside the thumb: page scroll.
+					if my < thumbStart {
+						h.ScrollUp(h.BufView().Height)
+					} else {
+						h.ScrollDown(h.BufView().Height)
+						h.ScrollAdjust()
+					}
+				}
+				return true
+			}
+		}
+	}
+
 	b := h.Buf
 	mx, my := e.Position()
 	// ignore click on the status line
@@ -109,6 +135,18 @@ func (h *BufPane) MousePress(e *tcell.EventMouse) bool {
 }
 
 func (h *BufPane) MouseDrag(e *tcell.EventMouse) bool {
+	if bw, ok := h.BWindow.(*display.BufWindow); ok && bw.IsScrollDragging() {
+		_, my := e.Position()
+		// Subtract the intra-thumb offset
+		effectiveY := my - bw.ScrollDragOffset()
+		targetLine := bw.ScrollPosFromMouse(effectiveY)
+		v := h.GetView()
+		v.StartLine = display.SLoc{targetLine, 0}
+		h.SetView(v)
+		h.ScrollAdjust()
+		return true
+	}
+
 	mx, my := e.Position()
 	// ignore drag on the status line
 	if my >= h.BufView().Y+h.BufView().Height {
@@ -130,6 +168,11 @@ func (h *BufPane) MouseDrag(e *tcell.EventMouse) bool {
 }
 
 func (h *BufPane) MouseRelease(e *tcell.EventMouse) bool {
+	if bw, ok := h.BWindow.(*display.BufWindow); ok && bw.IsScrollDragging() {
+		bw.EndScrollDrag()
+		return true
+	}
+
 	// We could finish the selection based on the release location as in the
 	// commented out code below, to allow text selections even in a terminal
 	// that doesn't support mouse motion events. But when the mouse click is

internal/display/bufwindow.go

@@ -29,6 +29,10 @@ type BufWindow struct {
 	hasMessage       bool
 	maxLineNumLength int
 	drawDivider      bool
+
+	scrollDragging bool
+	// Y delta between the mouse click position and the top of the thumb.
+	scrollDragOffset int
 }
 
 // NewBufWindow creates a new window at a location in the screen with a width and height
@@ -893,6 +897,72 @@ func (w *BufWindow) displayScrollBar() {
 	}
 }
 
+func (w *BufWindow) ScrollBarX() int {
+	if !w.Buf.Settings["scrollbar"].(bool) || w.Buf.LinesNum() <= w.Height {
+		return -1
+	}
+	return w.X + w.Width - 1
+}
+
+// returns the (inclusive) screen Y range [thumbStart, thumbEnd]
+// of the scrollbar thumb, or (-1,-1) when the scrollbar is not visible.
+func (w *BufWindow) ScrollBarBounds() (int, int) {
+	if !w.Buf.Settings["scrollbar"].(bool) || w.Buf.LinesNum() <= w.Height {
+		return -1, -1
+	}
+	barsize := int(float64(w.Height) / float64(w.Buf.LinesNum()) * float64(w.Height))
+	if barsize < 1 {
+		barsize = 1
+	}
+	barstart := w.Y + int(float64(w.StartLine.Line)/float64(w.Buf.LinesNum())*float64(w.Height))
+	barend := util.Min(barstart+barsize, w.Y+w.bufHeight) - 1
+	return barstart, barend
+}
+
+// returns the screen Y range [start, end] of the full
+// scrollbar track column (excluding the status line).
+func (w *BufWindow) ScrollBarTrackBounds() (int, int) {
+	if !w.Buf.Settings["scrollbar"].(bool) || w.Buf.LinesNum() <= w.Height {
+		return -1, -1
+	}
+	return w.Y, w.Y + w.bufHeight - 1
+}
+
+func (w *BufWindow) ScrollPosFromMouse(screenY int) int {
+	trackStart, trackEnd := w.ScrollBarTrackBounds()
+	trackH := trackEnd - trackStart + 1
+	if trackH <= 0 {
+		return 0
+	}
+	ratio := float64(screenY-trackStart) / float64(trackH)
+	line := int(ratio * float64(w.Buf.LinesNum()))
+	if line < 0 {
+		line = 0
+	}
+	if line >= w.Buf.LinesNum() {
+		line = w.Buf.LinesNum() - 1
+	}
+	return line
+}
+
+func (w *BufWindow) BeginScrollDrag(dragY, thumbStart int) {
+	w.scrollDragging = true
+	w.scrollDragOffset = dragY - thumbStart
+}
+
+func (w *BufWindow) EndScrollDrag() {
+	w.scrollDragging = false
+	w.scrollDragOffset = 0
+}
+
+func (w *BufWindow) IsScrollDragging() bool {
+	return w.scrollDragging
+}
+
+func (w *BufWindow) ScrollDragOffset() int {
+	return w.scrollDragOffset
+}
+
 // Display displays the buffer and the statusline
 func (w *BufWindow) Display() {
 	w.updateDisplayInfo()