Support preview scroll offset relative to window height

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

CHANGELOG.md

@@ -1,6 +1,16 @@
 CHANGELOG
 =========
 
+0.22.1
+------
+- Support preview scroll offset relative to window height
+  ```sh
+  git grep --line-number '' |
+    fzf --delimiter : \
+        --preview 'bat --style=numbers --color=always --highlight-line {2} {1}' \
+        --preview-window +{2}-/2
+  ```
+
 0.22.0
 ------
 - Added more options for `--bind`

man/man1/fzf.1

@@ -394,7 +394,8 @@ execute the command in the background.
 window. \fBSCROLL\fR can be either a numeric integer or a single-field index
 expression that refers to a numeric integer. The optional \fB-OFFSET\fR part is
 for adjusting the base offset so that you can see the text above it. It should
-be given as a numeric integer.
+be given as a numeric integer (\fB-INTEGER\fR), or as a denominator form
+(\fB-/INTEGER\fR) for specifying a fraction of the preview window height.
 
 .RS
 .B POSITION: (default: right)
@@ -411,9 +412,15 @@ e.g.
      fzf --preview="file {}" --preview-window=down:1
 
      # Initial scroll offset is set to the line number of each line of
-     # git grep output *minus* 5 lines
+     # git grep output *minus* 5 lines (-5)
      git grep --line-number '' |
-       fzf --delimiter : --preview 'nl {1}' --preview-window +{2}-5\fR
+       fzf --delimiter : --preview 'nl {1}' --preview-window +{2}-5
+
+     # Preview with bat, matching line in the middle of the window (-/2)
+     git grep --line-number '' |
+       fzf --delimiter : \\
+           --preview 'bat --style=numbers --color=always --highlight-line {2} {1}' \\
+           --preview-window +{2}-/2\fR
 
 .RE
 .SS Scripting

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 "":

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)