Add offset-up and offset-down

# Scrolling will behave similarly to CTRL-E and CTRL-Y of vim
  fzf --bind scroll-up:offset-up,scroll-down:offset-down \
      --bind ctrl-y:offset-up,ctrl-e:offset-down \
      --scroll-off=5

Close #3456

CHANGELOG.md

@@ -55,7 +55,15 @@ CHANGELOG
         --bind 'preview-scroll-down:preview-down+preview-down' \
         --preview 'cat {}'
     ```
+- Added `offset-up` and `offset-down` actions
+  ```sh
+  # Scrolling will behave similarly to CTRL-E and CTRL-Y of vim
+  fzf --bind scroll-up:offset-up,scroll-down:offset-down \
+      --bind ctrl-y:offset-up,ctrl-e:offset-down \
+      --scroll-off=5
+  ```
 - Shell extensions
+    - Updated bash completion for fzf options
     - bash key bindings no longer requires perl; it will use awk or mawk
       instead if perl is not found
     - Basic context-aware completion for ssh command

man/man1/fzf.1

@@ -1157,6 +1157,8 @@ A key or an event can be bound to one or more of the following actions.
     \fBhalf-page-down\fR
     \fBhalf-page-up\fR
     \fBhide-preview\fR
+    \fBoffset-down\fR                  (similar to CTRL-E of Vim)
+    \fBoffset-up\fR                    (similar to CTRL-Y of Vim)
     \fBpos(...)\fR                     (move cursor to the numeric position; negative number to count from the end)
     \fBprev-history\fR                 (\fIctrl-p\fR on \fB--history\fR)
     \fBprev-selected\fR                (move to the previous selected item)

src/options.go

@@ -1179,6 +1179,10 @@ func parseActionList(masked string, original string, prevActions []*action, putA
 			appendAction(actTogglePreviewWrap)
 		case "toggle-sort":
 			appendAction(actToggleSort)
+		case "offset-up":
+			appendAction(actOffsetUp)
+		case "offset-down":
+			appendAction(actOffsetDown)
 		case "preview-top":
 			appendAction(actPreviewTop)
 		case "preview-bottom":

src/terminal.go

@@ -378,6 +378,8 @@ const (
 	actPosition
 	actHalfPageUp
 	actHalfPageDown
+	actOffsetUp
+	actOffsetDown
 	actJump
 	actJumpAccept
 	actPrintQuery
@@ -3404,6 +3406,19 @@ func (t *Terminal) Loop() {
 			case actHalfPageDown:
 				t.vmove(-(t.maxItems() / 2), false)
 				req(reqList)
+			case actOffsetUp, actOffsetDown:
+				diff := 1
+				if a.t == actOffsetDown {
+					diff = -1
+				}
+				t.offset += diff
+				before := t.offset
+				t.constrain()
+				if before != t.offset {
+					t.offset = before
+					t.vmove(diff, false)
+				}
+				req(reqList)
 			case actJump:
 				t.jumping = jumpEnabled
 				req(reqJump)