Fix preview follow/scroll with long wrapped lines

Fixes bugs reported in https://github.com/junegunn/fzf/pull/4703:

* Clamp followOffset return value to avoid going past the end of lines
* Account for t.previewed.filled when determining scrollability

src/terminal.go

@@ -4223,7 +4223,7 @@ func (t *Terminal) followOffset() int {
 	for i := len(body) - 1; i >= 0; i-- {
 		h := t.previewLineHeight(body[i], maxWidth)
 		if visualLines+h > height {
-			return headerLines + i + 1
+			return min(len(lines)-1, headerLines+i+1)
 		}
 		visualLines += h
 	}
@@ -4521,7 +4521,7 @@ Loop:
 				}
 			}
 
-			t.previewer.scrollable = t.previewer.scrollable || t.pwindow.Y() == height-1 && t.pwindow.X() == t.pwindow.Width()
+			t.previewer.scrollable = t.previewer.scrollable || t.pwindow.Y() == height-1 && t.pwindow.X() == t.pwindow.Width() || t.previewed.filled
 			if fillRet == tui.FillNextLine {
 				continue
 			} else if fillRet == tui.FillSuspend {
@@ -4544,7 +4544,7 @@ Loop:
 		}
 		lineNo++
 	}
-	t.previewer.scrollable = t.previewer.scrollable || index < len(lines)-1
+	t.previewer.scrollable = t.previewer.scrollable || t.previewed.filled || index < len(lines)-1
 	t.previewed.image = image
 	t.previewed.wireframe = wireframe
 }

test/test_preview.rb

@@ -393,6 +393,20 @@ class TestPreview < TestInteractive
     end
   end
 
+  def test_preview_follow_wrap_long_line
+    tmux.send_keys %(seq 1 | #{FZF} --preview "seq 2; yes yes | head -10000 | tr '\n' ' '" --preview-window follow,wrap --bind up:preview-up,down:preview-down), :Enter
+    tmux.until do |lines|
+      assert_equal 1, lines.match_count
+      assert lines.any_include?('3/3 │')
+    end
+    tmux.send_keys :Up
+    tmux.until { |lines| assert lines.any_include?('2/3 │') }
+    tmux.send_keys :Up
+    tmux.until { |lines| assert lines.any_include?('1/3 │') }
+    tmux.send_keys :Down
+    tmux.until { |lines| assert lines.any_include?('2/3 │') }
+  end
+
   def test_close
     tmux.send_keys "seq 100 | #{FZF} --preview 'echo foo' --bind ctrl-c:close", :Enter
     tmux.until { |lines| assert_equal 100, lines.match_count }