Resume preview following if the user scrolls the window to the bottom

3 files changed, 111 insertions, 10 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 718c9326..04b276f2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -109,6 +109,11 @@ CHANGELOG
       color for `border`. Same holds true for `scrollbar`. This is to reduce
       the number of configuration items required to achieve a consistent color
       scheme.
+    - If `follow` flag is specified in `--preview-window` option, fzf will
+      automatically scroll to the bottom of the streaming preview output. But
+      when the user manually scrolls the window, the following stops. With
+      this version, fzf will resume following if the user scrolls the window
+      to the bottom.
 - Added color name `preview-label` for `--preview-label` (defaults to `label`
   for `--border-label`)
 - Minor bug fixes and improvements
diff --git a/src/terminal.go b/src/terminal.go
index 80c9e1a1..8cd9a143 100644
--- a/src/terminal.go
+++ b/src/terminal.go
@@ -68,6 +68,38 @@ const (
 	jumpAcceptEnabled
 )
 
+type resumableState int
+
+const (
+	disabledState resumableState = iota
+	pausedState
+	enabledState
+)
+
+func (s resumableState) Enabled() bool {
+	return s == enabledState
+}
+
+func (s *resumableState) Force(flag bool) {
+	if flag {
+		*s = enabledState
+	} else {
+		*s = disabledState
+	}
+}
+
+func (s *resumableState) Set(flag bool) {
+	if *s == disabledState {
+		return
+	}
+
+	if flag {
+		*s = enabledState
+	} else {
+		*s = pausedState
+	}
+}
+
 type previewer struct {
 	version    int64
 	lines      []string
@@ -75,7 +107,7 @@ type previewer struct {
 	enabled    bool
 	scrollable bool
 	final      bool
-	following  bool
+	following  resumableState
 	spinner    string
 	bar        []bool
 }
@@ -601,7 +633,7 @@ func NewTerminal(opts *Options, eventBox *util.EventBox) *Terminal {
 		reqBox:             util.NewEventBox(),
 		initialPreviewOpts: opts.Preview,
 		previewOpts:        opts.Preview,
-		previewer:          previewer{0, []string{}, 0, len(opts.Preview.command) > 0, false, true, false, "", []bool{}},
+		previewer:          previewer{0, []string{}, 0, len(opts.Preview.command) > 0, false, true, disabledState, "", []bool{}},
 		previewed:          previewed{0, 0, 0, false},
 		previewBox:         previewBox,
 		eventBox:           eventBox,
@@ -2639,12 +2671,15 @@ func (t *Terminal) Loop() {
 						result := value.(previewResult)
 						if t.previewer.version != result.version {
 							t.previewer.version = result.version
-							t.previewer.following = t.previewOpts.follow
+							t.previewer.following.Force(t.previewOpts.follow)
+							if t.previewer.following.Enabled() {
+								t.previewer.offset = 0
+							}
 						}
 						t.previewer.lines = result.lines
 						t.previewer.spinner = result.spinner
-						if t.previewer.following {
-							t.previewer.offset = len(t.previewer.lines) - t.pwindow.Height()
+						if t.previewer.following.Enabled() {
+							t.previewer.offset = util.Max(t.previewer.offset, len(t.previewer.lines)-(t.pwindow.Height()-t.previewOpts.headerLines))
 						} else if result.offset >= 0 {
 							t.previewer.offset = util.Constrain(result.offset, t.previewOpts.headerLines, len(t.previewer.lines)-1)
 						}
@@ -2741,7 +2776,6 @@ func (t *Terminal) Loop() {
 			if !t.previewer.scrollable {
 				return
 			}
-			t.previewer.following = false
 			numLines := len(t.previewer.lines)
 			headerLines := t.previewOpts.headerLines
 			if t.previewOpts.cycle {
@@ -2751,6 +2785,7 @@ func (t *Terminal) Loop() {
 			newOffset = util.Constrain(newOffset, headerLines, numLines-1)
 			if t.previewer.offset != newOffset {
 				t.previewer.offset = newOffset
+				t.previewer.following.Set(t.previewer.offset >= numLines-(t.pwindow.Height()-headerLines))
 				req(reqPreviewRefresh)
 			}
 		}
@@ -3177,7 +3212,7 @@ func (t *Terminal) Loop() {
 						y = util.Constrain(y, 0, effectiveHeight-barLength)
 						// offset = (total - maxItems) * barStart / (maxItems - barLength)
 						t.previewer.offset = headerLines + int(math.Ceil(float64(y)*float64(numLines-effectiveHeight)/float64(effectiveHeight-barLength)))
-						t.previewer.following = false
+						t.previewer.following.Set(t.previewer.offset >= numLines-effectiveHeight)
 						req(reqPreviewRefresh)
 					}
 					break
@@ -3322,7 +3357,7 @@ func (t *Terminal) Loop() {
 				}
 
 				// Resume following
-				t.previewer.following = t.previewOpts.follow
+				t.previewer.following.Force(t.previewOpts.follow)
 			case actNextSelected, actPrevSelected:
 				if len(t.selected) > 0 {
 					total := t.merger.Length()
diff --git a/test/test_go.rb b/test/test_go.rb
index 8031e81e..60322e15 100755
--- a/test/test_go.rb
+++ b/test/test_go.rb
@@ -1943,8 +1943,69 @@ class TestGoFZF < TestBase
   end
 
   def test_preview_window_follow
-    tmux.send_keys "#{FZF} --preview 'seq 1000 | nl' --preview-window down:noborder:follow", :Enter
-    tmux.until { |lines| assert_equal '1000  1000', lines[-1].strip }
+    file = Tempfile.new('fzf-follow')
+    file.sync = true
+
+    tmux.send_keys %[seq 100 | #{FZF} --preview 'tail -f "#{file.path}"' --preview-window follow --bind 'up:preview-up,down:preview-down,space:change-preview-window:follow|nofollow' --preview-window '~3'], :Enter
+    tmux.until { |lines| lines.item_count == 100 }
+
+    # Write to the temporary file, and check if the preview window is showing
+    # the last line of the file
+    3.times { file.puts _1 } # header lines
+    1000.times { file.puts _1 }
+    tmux.until { |lines| assert_includes lines[1], '/1003' }
+    tmux.until { |lines| assert_includes lines[-2], '999' }
+
+    # Scroll the preview window and fzf should stop following the file content
+    tmux.send_keys :Up
+    tmux.until { |lines| assert_includes lines[-2], '998' }
+    file.puts 'foo', 'bar'
+    tmux.until do |lines|
+      assert_includes lines[1], '/1005'
+      assert_includes lines[-2], '998'
+    end
+
+    # Scroll back to the bottom and fzf should start following the file again
+    %w[999 foo bar].each do |item|
+      wait do
+        tmux.send_keys :Down
+        tmux.until { |lines| assert_includes lines[-2], item }
+      end
+    end
+    file.puts 'baz'
+    tmux.until do |lines|
+      assert_includes lines[1], '/1006'
+      assert_includes lines[-2], 'baz'
+    end
+
+    # Scroll upwards to stop following
+    tmux.send_keys :Up
+    wait { |line| assert_includes lines[-2], 'bar' }
+    file.puts 'aaa'
+    tmux.until do |lines|
+      assert_includes lines[1], '/1007'
+      assert_includes lines[-2], 'bar'
+    end
+
+    # Manually enable following
+    tmux.send_keys :Space
+    tmux.until { |lines| assert_includes lines[-2], 'aaa' }
+    file.puts 'bbb'
+    tmux.until do |lines|
+      assert_includes lines[1], '/1008'
+      assert_includes lines[-2], 'bbb'
+    end
+
+    # Disable following
+    tmux.send_keys :Space
+    file.puts 'ccc', 'ddd'
+    tmux.until do |lines|
+      assert_includes lines[1], '/1010'
+      assert_includes lines[-2], 'bbb'
+    end
+  rescue
+    file.close
+    file.unlink
   end
 
   def test_toggle_preview_wrap