Support ANSI escape sequence for clearing display in preview window

  fzf --preview 'for i in $(seq 100000); do
    (( i % 200 == 0 )) && printf "\033[2J"
    echo "$i"
    sleep 0.01
  done'

4 files changed, 45 insertions, 2 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 75595df0..c11585ef 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,9 +3,27 @@ CHANGELOG
 
 0.24.0
 ------
-- fzf can render preview window before the command completes
+- Real-time rendering of preview window
   ```sh
+  # fzf can render preview window before the command completes
   fzf --preview 'sleep 1; for i in $(seq 100); do echo $i; sleep 0.01; done'
+
+  # Preview window can process ANSI escape sequence (CSI 2 J) for clearing the display
+  fzf --preview 'for i in $(seq 100000); do
+    (( i % 200 == 0 )) && printf "\033[2J"
+    echo "$i"
+    sleep 0.01
+  done'
+  ```
+- To indicate if `--multi` mode is enabled, fzf will print the number of
+  selected items even when no item is selected
+  ```sh
+  seq 100 | fzf
+    # 100/100
+  seq 100 | fzf --multi
+    # 100/100 (0)
+  seq 100 | fzf --multi 5
+    # 100/100 (0/5)
   ```
 
 0.23.1
diff --git a/man/man1/fzf.1 b/man/man1/fzf.1
index d0e27908..f48c8424 100644
--- a/man/man1/fzf.1
+++ b/man/man1/fzf.1
@@ -379,6 +379,18 @@ Note that you can escape a placeholder pattern by prepending a backslash.
 
 Preview window will be updated even when there is no match for the current
 query if any of the placeholder expressions evaluates to a non-empty string.
+
+Since 0.24.0, fzf can render partial preview content before the preview command
+completes. ANSI escape sequence for clearing the display (\fBCSI 2 J\fR) is
+supported, so you can use it to implement preview window that is constantly
+updating.
+
+e.g.
+      \fBfzf --preview 'for i in $(seq 100000); do
+        (( i % 200 == 0 )) && printf "\\033[2J"
+        echo "$i"
+        sleep 0.01
+      done'\fR
 .RE
 .TP
 .BI "--preview-window=" "[POSITION][:SIZE[%]][:rounded|sharp|noborder][:[no]wrap][:[no]cycle][:[no]hidden][:+SCROLL[-OFFSET]][:default]"
@@ -389,7 +401,6 @@ query if any of the placeholder expressions evaluates to a non-empty string.
     \fBdown
     \fBleft
     \fBright
-.RE
 
 \fRDetermines the layout of the preview window. If the argument contains
 \fB:hidden\fR, the preview window will be hidden by default until
diff --git a/src/terminal.go b/src/terminal.go
index e1c5c7bf..17a4377f 100644
--- a/src/terminal.go
+++ b/src/terminal.go
@@ -26,6 +26,7 @@ var numericPrefix *regexp.Regexp
 var activeTempFiles []string
 
 const ellipsis string = ".."
+const clearCode string = "\x1b[2J"
 
 func init() {
 	placeholder = regexp.MustCompile(`\\?(?:{[+sf]*[0-9,-.]*}|{q}|{\+?f?nf?})`)
@@ -1834,6 +1835,13 @@ func (t *Terminal) Loop() {
 									line := eachLine.line
 									err := eachLine.err
 									if len(line) > 0 {
+										clearIndex := strings.Index(line, clearCode)
+										if clearIndex >= 0 {
+											lines = []string{}
+											line = line[clearIndex+len(clearCode):]
+											version--
+											offset = 0
+										}
 										lines = append(lines, line)
 									}
 									if err != nil {
diff --git a/test/test_go.rb b/test/test_go.rb
index 2861235e..3ff118c1 100755
--- a/test/test_go.rb
+++ b/test/test_go.rb
@@ -1817,6 +1817,12 @@ class TestGoFZF < TestBase
     assert_equal %w[A Á], `#{echoes} | #{FZF} -f A`.lines.map(&:chomp)
     assert_equal %w[Á], `#{echoes} | #{FZF} -f Á`.lines.map(&:chomp)
   end
+
+  def test_preview_clear_screen
+    tmux.send_keys %{seq 100 | #{FZF} --preview 'for i in $(seq 300); do (( i % 200 == 0 )) && printf "\\033[2J"; echo "[$i]"; sleep 0.001; done'}, :Enter
+    tmux.until { |lines| lines.item_count == 100 }
+    tmux.until { |lines| lines[1]&.include?('[200]') }
+  end
 end
 
 module TestShell