Implement --scroll-off=LINES

Close #2533

src/options.go

@@ -44,8 +44,10 @@ const usage = `usage: fzf [options]
     --bind=KEYBINDS       Custom key bindings. Refer to the man page.
     --cycle               Enable cyclic scroll
     --keep-right          Keep the right end of the line visible on overflow
+    --scroll-off=LINES    Number of screen lines to keep above or below when
+                          scrolling to the top or to the bottom (default: 0)
     --no-hscroll          Disable horizontal scroll
-    --hscroll-off=COL     Number of screen columns to keep to the right of the
+    --hscroll-off=COLS    Number of screen columns to keep to the right of the
                           highlighted substring (default: 10)
     --filepath-word       Make word-wise movements respect path separators
     --jump-labels=CHARS   Label characters for jump and jump-accept
@@ -200,6 +202,7 @@ type Options struct {
 	KeepRight   bool
 	Hscroll     bool
 	HscrollOff  int
+	ScrollOff   int
 	FileWord    bool
 	InfoStyle   infoStyle
 	JumpLabels  string
@@ -261,6 +264,7 @@ func defaultOptions() *Options {
 		KeepRight:   false,
 		Hscroll:     true,
 		HscrollOff:  10,
+		ScrollOff:   0,
 		FileWord:    false,
 		InfoStyle:   infoDefault,
 		JumpLabels:  defaultJumpLabels,
@@ -1354,6 +1358,8 @@ func parseOptions(opts *Options, allArgs []string) {
 			opts.Hscroll = false
 		case "--hscroll-off":
 			opts.HscrollOff = nextInt(allArgs, &i, "hscroll offset required")
+		case "--scroll-off":
+			opts.ScrollOff = nextInt(allArgs, &i, "scroll offset required")
 		case "--filepath-word":
 			opts.FileWord = true
 		case "--no-filepath-word":
@@ -1530,6 +1536,8 @@ func parseOptions(opts *Options, allArgs []string) {
 				opts.Tabstop = atoi(value)
 			} else if match, value := optString(arg, "--hscroll-off="); match {
 				opts.HscrollOff = atoi(value)
+			} else if match, value := optString(arg, "--scroll-off="); match {
+				opts.ScrollOff = atoi(value)
 			} else if match, value := optString(arg, "--jump-labels="); match {
 				opts.JumpLabels = value
 				validateJumpLabels = true
@@ -1547,6 +1555,10 @@ func parseOptions(opts *Options, allArgs []string) {
 		errorExit("hscroll offset must be a non-negative integer")
 	}
 
+	if opts.ScrollOff < 0 {
+		errorExit("scroll offset must be a non-negative integer")
+	}
+
 	if opts.Tabstop < 1 {
 		errorExit("tab stop must be a positive integer")
 	}

src/terminal.go

@@ -121,6 +121,7 @@ type Terminal struct {
 	keepRight    bool
 	hscroll      bool
 	hscrollOff   int
+	scrollOff    int
 	wordRubout   string
 	wordNext     string
 	cx           int
@@ -502,6 +503,7 @@ func NewTerminal(opts *Options, eventBox *util.EventBox) *Terminal {
 		keepRight:   opts.KeepRight,
 		hscroll:     opts.Hscroll,
 		hscrollOff:  opts.HscrollOff,
+		scrollOff:   opts.ScrollOff,
 		wordRubout:  wordRubout,
 		wordNext:    wordNext,
 		cx:          len(input),
@@ -2749,9 +2751,26 @@ func (t *Terminal) constrain() {
 
 	t.cy = util.Constrain(t.cy, 0, count-1)
 
-	minOffset := t.cy - height + 1
+	minOffset := util.Max(t.cy-height+1, 0)
 	maxOffset := util.Max(util.Min(count-height, t.cy), 0)
 	t.offset = util.Constrain(t.offset, minOffset, maxOffset)
+	if t.scrollOff == 0 {
+		return
+	}
+
+	scrollOff := util.Min(height/2, t.scrollOff)
+	for {
+		prevOffset := t.offset
+		if t.cy-t.offset < scrollOff {
+			t.offset = util.Max(minOffset, t.offset-1)
+		}
+		if t.cy-t.offset >= height-scrollOff {
+			t.offset = util.Min(maxOffset, t.offset+1)
+		}
+		if t.offset == prevOffset {
+			break
+		}
+	}
 }
 
 func (t *Terminal) vmove(o int, allowCycle bool) {