(Experimental) Add support for iTerm2 inline image protocol

Close #1102

  fzf --preview 'imgcat -W $FZF_PREVIEW_COLUMNS -H $FZF_PREVIEW_LINES {}'

Notes:
* There is no good way to determine the height of the rendered image,
  so we assume that the image takes the full height of the preview
  window. So the image cannot be displayed with the other text.
* fzf-preview.sh script was updated to use `imgcat` if it's available
  but `chafa` is not.
* iTerm2 also supports Sixel, so adding support for this protocol is not
  quite necessary but it renders animated GIFs much better (e.g. looping).

src/terminal.go

@@ -66,7 +66,8 @@ func init() {
 	// * https://github.com/tmux/tmux/wiki/FAQ#what-is-the-passthrough-escape-sequence-and-how-do-i-use-it
 	// * https://sw.kovidgoyal.net/kitty/graphics-protocol
 	// * https://en.wikipedia.org/wiki/Sixel
-	passThroughRegex = regexp.MustCompile(`\x1bPtmux;\x1b\x1b.*?[^\x1b]\x1b\\|\x1b(_G|P[0-9;]*q).*?\x1b\\\r?`)
+	// * https://iterm2.com/documentation-images.html
+	passThroughRegex = regexp.MustCompile(`\x1bPtmux;\x1b\x1b.*?[^\x1b]\x1b\\|\x1b(_G|P[0-9;]*q).*?\x1b\\\r?|\x1b]1337;.*?\a`)
 }
 
 type jumpMode int
@@ -125,7 +126,7 @@ type previewed struct {
 	numLines  int
 	offset    int
 	filled    bool
-	sixel     bool
+	image     bool
 	wipe      bool
 	wireframe bool
 }
@@ -2027,7 +2028,7 @@ func (t *Terminal) renderPreviewText(height int, lines []string, lineNo int, unc
 	var ansi *ansiState
 	spinnerRedraw := t.pwindow.Y() == 0
 	wiped := false
-	sixel := false
+	image := false
 	wireframe := false
 Loop:
 	for _, line := range lines {
@@ -2055,18 +2056,25 @@ Loop:
 				t.pwindow.Move(y, x)
 			}
 			for idx, passThrough := range passThroughs {
-				// Handling Sixel output
+				// Handling Sixel/iTerm image
 				requiredLines := 0
 				isSixel := strings.HasPrefix(passThrough, "\x1bP")
-				if isSixel {
+				isItermImage := strings.HasPrefix(passThrough, "\x1b]1337;")
+				isImage := isSixel || isItermImage
+				if isImage {
 					t.previewed.wipe = true
-					if t.termSize.PxHeight > 0 {
+					// NOTE: We don't have a good way to get the height of an iTerm image,
+					// so we assume that it requires the full height of the preview
+					// window.
+					requiredLines = height
+
+					if isSixel && t.termSize.PxHeight > 0 {
 						rows := strings.Count(passThrough, "-")
 						requiredLines = int(math.Ceil(float64(rows*6*t.termSize.Lines) / float64(t.termSize.PxHeight)))
 					}
 				}
 
-				// Render wireframe when Sixel image cannot be displayed entirely
+				// Render wireframe when the image cannot be displayed entirely
 				if requiredLines > 0 && y+requiredLines > height {
 					top := true
 					for ; y < height; y++ {
@@ -2081,17 +2089,17 @@ Loop:
 				}
 
 				// Clear previous wireframe or any other text
-				if (t.previewed.wireframe || isSixel && !t.previewed.sixel) && !wiped {
+				if (t.previewed.wireframe || isImage && !t.previewed.image) && !wiped {
 					wiped = true
 					for i := y + 1; i < height; i++ {
 						t.pwindow.MoveAndClear(i, 0)
 					}
 					// Required for tcell to clear the previous text
-					if !t.previewed.sixel {
+					if !t.previewed.image {
 						t.tui.Sync(false)
 					}
 				}
-				sixel = sixel || isSixel
+				image = image || isImage
 				if idx == 0 {
 					t.pwindow.MoveAndClear(y, x)
 				} else {
@@ -2154,7 +2162,7 @@ Loop:
 		}
 		lineNo++
 	}
-	t.previewed.sixel = sixel
+	t.previewed.image = image
 	t.previewed.wireframe = wireframe
 }