Support preview scroll offset relative to window height

Related: https://github.com/junegunn/fzf.vim/issues/1092

2 files changed, 12 insertions, 4 deletions

diff --git a/src/options.go b/src/options.go
index 699e8a55..a916e806 100644
--- a/src/options.go
+++ b/src/options.go
@@ -995,7 +995,7 @@ func parsePreviewWindow(opts *previewOpts, input string) {
 
 	tokens := strings.Split(input, ":")
 	sizeRegex := regexp.MustCompile("^[1-9][0-9]*%?$")
-	offsetRegex := regexp.MustCompile("^\\+([0-9]+|{-?[0-9]+})(-[0-9]+)?$")
+	offsetRegex := regexp.MustCompile("^\\+([0-9]+|{-?[0-9]+})(-[0-9]+|-/[1-9][0-9]*)?$")
 	for _, token := range tokens {
 		switch token {
 		case "":
diff --git a/src/terminal.go b/src/terminal.go
index a7e9e9b2..2fa13984 100644
--- a/src/terminal.go
+++ b/src/terminal.go
@@ -1374,7 +1374,7 @@ func atopi(s string) int {
 	return n
 }
 
-func (t *Terminal) evaluateScrollOffset(list []*Item) int {
+func (t *Terminal) evaluateScrollOffset(list []*Item, height int) int {
 	offsetExpr := t.replacePlaceholder(t.preview.scroll, false, "", list)
 	nums := strings.Split(offsetExpr, "-")
 	switch len(nums) {
@@ -1387,6 +1387,13 @@ func (t *Terminal) evaluateScrollOffset(list []*Item) int {
 		} else if len(nums) == 1 {
 			return base - 1
 		}
+		if nums[1][0] == '/' {
+			denom := atopi(nums[1][1:])
+			if denom == 0 {
+				return base
+			}
+			return base - height/denom
+		}
 		return base - atopi(nums[1]) - 1
 	default:
 		return 0
@@ -1676,11 +1683,12 @@ func (t *Terminal) Loop() {
 				// We don't display preview window if no match
 				if items[0] != nil {
 					command := t.replacePlaceholder(commandTemplate, false, string(t.Input()), items)
-					offset := t.evaluateScrollOffset(items)
+					height := t.pwindow.Height()
+					offset := t.evaluateScrollOffset(items, height)
 					cmd := util.ExecCommand(command, true)
 					if t.pwindow != nil {
 						env := os.Environ()
-						lines := fmt.Sprintf("LINES=%d", t.pwindow.Height())
+						lines := fmt.Sprintf("LINES=%d", height)
 						columns := fmt.Sprintf("COLUMNS=%d", t.pwindow.Width())
 						env = append(env, lines)
 						env = append(env, "FZF_PREVIEW_"+lines)