Implement streaming preview window (junegunn#2215)

Fix junegunn#2212

    # Will start rendering after 200ms, update every 100ms
    fzf --preview 'for i in $(seq 100); do echo $i; sleep 0.01; done'

    # Should print "Loading .." message after 500ms
    fzf --preview 'sleep 1; for i in $(seq 100); do echo $i; sleep 0.01; done'

    # The first line should appear after 200ms
    fzf --preview 'date; sleep 2; date'

    # Should not render before enough lines for the scroll offset are ready
    rg --line-number --no-heading --color=always ^ |
      fzf --delimiter : --ansi --preview-window '+{2}-/2' \
          --preview 'sleep 1; bat --style=numbers --color=always --pager=never --highlight-line={2} {1}'

CHANGELOG.md

@@ -1,6 +1,13 @@
 CHANGELOG
 =========
 
+0.24.0
+------
+- fzf can render preview window before the command completes
+  ```sh
+  fzf --preview 'sleep 1; for i in $(seq 100); do echo $i; sleep 0.01; done'
+  ```
+
 0.23.1
 ------
 - Added `--preview-window` options for disabling flags

README.md

@@ -582,26 +582,18 @@ and fzf will warn you about it. To suppress the warning message, we added
 
 ### Preview window
 
-When the `--preview` option is set, fzf automatically starts an external process 
-with the current line as the argument and shows the result in the split window. 
-Your `$SHELL` is used to execute the command with `$SHELL -c COMMAND`. 
+When the `--preview` option is set, fzf automatically starts an external process
+with the current line as the argument and shows the result in the split window.
+Your `$SHELL` is used to execute the command with `$SHELL -c COMMAND`.
 The window can be scrolled using the mouse or custom key bindings.
 
 ```bash
 # {} is replaced to the single-quoted string of the focused line
 fzf --preview 'cat {}'
 ```
 
-Since the preview window is updated only after the process is complete, it's
-important that the command finishes quickly.
-
-```bash
-# Use head instead of cat so that the command doesn't take too long to finish
-fzf --preview 'head -100 {}'
-```
-
 Preview window supports ANSI colors, so you can use any program that
-syntax-highlights the content of a file, such as 
+syntax-highlights the content of a file, such as
 [Bat](https://github.com/sharkdp/bat) or
 [Highlight](http://www.andre-simon.de/doku/highlight/en/highlight.php):
 

src/constants.go

@@ -27,6 +27,8 @@ const (
 	initialDelayTac   = 100 * time.Millisecond
 	spinnerDuration   = 100 * time.Millisecond
 	previewCancelWait = 500 * time.Millisecond
+	previewChunkDelay = 100 * time.Millisecond
+	previewDelayed    = 500 * time.Millisecond
 	maxPatternLength  = 300
 	maxMulti          = math.MaxInt32