Add ScrollOffMargin user config

When set to a non-zero value, views will scroll when the selection gets this
close to the top or bottom of the view.

pkg/gui/controllers/scroll_off_margin.go

@@ -0,0 +1,70 @@
+package controllers
+
+import (
+	"github.com/jesseduffield/lazygit/pkg/gui/types"
+	"github.com/jesseduffield/lazygit/pkg/utils"
+)
+
+// To be called after pressing up-arrow; checks whether the cursor entered the
+// top scroll-off margin, and so the view needs to be scrolled up one line
+func checkScrollUp(view types.IViewTrait, scrollOffMargin int, lineIdxBefore int, lineIdxAfter int) {
+	viewPortStart, viewPortHeight := view.ViewPortYBounds()
+
+	linesToScroll := calculateLinesToScrollUp(
+		viewPortStart, viewPortHeight, scrollOffMargin, lineIdxBefore, lineIdxAfter)
+	if linesToScroll != 0 {
+		view.ScrollUp(linesToScroll)
+	}
+}
+
+// To be called after pressing down-arrow; checks whether the cursor entered the
+// bottom scroll-off margin, and so the view needs to be scrolled down one line
+func checkScrollDown(view types.IViewTrait, scrollOffMargin int, lineIdxBefore int, lineIdxAfter int) {
+	viewPortStart, viewPortHeight := view.ViewPortYBounds()
+
+	linesToScroll := calculateLinesToScrollDown(
+		viewPortStart, viewPortHeight, scrollOffMargin, lineIdxBefore, lineIdxAfter)
+	if linesToScroll != 0 {
+		view.ScrollDown(linesToScroll)
+	}
+}
+
+func calculateLinesToScrollUp(viewPortStart int, viewPortHeight int, scrollOffMargin int, lineIdxBefore int, lineIdxAfter int) int {
+	// Cap the margin to half the view height. This allows setting the config to
+	// a very large value to keep the cursor always in the middle of the screen.
+	// Use +.5 so that if the height is even, the top margin is one line higher
+	// than the bottom margin.
+	scrollOffMargin = utils.Min(scrollOffMargin, int((float64(viewPortHeight)+.5)/2))
+
+	// Scroll only if the "before" position was visible (this could be false if
+	// the scroll wheel was used to scroll the selected line out of view) ...
+	if lineIdxBefore >= viewPortStart && lineIdxBefore < viewPortStart+viewPortHeight {
+		marginEnd := viewPortStart + scrollOffMargin
+		// ... and the "after" position is within the top margin (or before it)
+		if lineIdxAfter < marginEnd {
+			return marginEnd - lineIdxAfter
+		}
+	}
+
+	return 0
+}
+
+func calculateLinesToScrollDown(viewPortStart int, viewPortHeight int, scrollOffMargin int, lineIdxBefore int, lineIdxAfter int) int {
+	// Cap the margin to half the view height. This allows setting the config to
+	// a very large value to keep the cursor always in the middle of the screen.
+	// Use -.5 so that if the height is even, the bottom margin is one line lower
+	// than the top margin.
+	scrollOffMargin = utils.Min(scrollOffMargin, int((float64(viewPortHeight)-.5)/2))
+
+	// Scroll only if the "before" position was visible (this could be false if
+	// the scroll wheel was used to scroll the selected line out of view) ...
+	if lineIdxBefore >= viewPortStart && lineIdxBefore < viewPortStart+viewPortHeight {
+		marginStart := viewPortStart + viewPortHeight - scrollOffMargin - 1
+		// ... and the "after" position is within the bottom margin (or after it)
+		if lineIdxAfter > marginStart {
+			return lineIdxAfter - marginStart
+		}
+	}
+
+	return 0
+}