Keep rxvt keys intact when checking the frame

- Leave a short frame the parser rejects as it was. rxvt sends keys of that
  size fzf does not know, such as \e[3^ for CTRL-DELETE, and reporting them
  as ALT-[ typed "3^" into the query
- Do not widen a parse that ends in an intermediate byte. rxvt ends keys with
  $, as in \e[7$ for SHIFT-HOME, and widening took the next typed key with it

src/tui/light.go

@@ -348,6 +348,8 @@ func getEnv(name string, defaultValue int) int {
 func csiContinues(b byte) bool { return b >= 0x20 && b <= 0x3f }
 func csiFinal(b byte) bool     { return b >= 0x40 && b <= 0x7e }
 
+func csiIntermediate(b byte) bool { return b >= 0x20 && b <= 0x2f }
+
 // csiEnd returns the length of the CSI sequence at the start of the buffer, or
 // 0 if it has no final byte yet or is malformed.
 func csiEnd(buffer []byte) int {
@@ -645,18 +647,15 @@ func (r *LightRenderer) escSequence(sz *int) Event {
 	if end == 0 {
 		return ev
 	}
-	// The same rule as the fall-through: four bytes or fewer could be ALT-[
-	// or ALT-O and typed text
+	// Four bytes or fewer could be ALT-[ or ALT-O and typed text, and rxvt sends
+	// keys of that size fzf does not know, such as \e[3^ for CTRL-DELETE
 	if end <= 4 {
-		if ev.Type == Invalid {
-			*sz = 2
-			return AltKey(rune(r.buffer[1]))
-		}
 		return ev
 	}
-	// A key matched on a prefix, such as Home for \e[70;5u, or a sequence
-	// given up on partway
-	if end > *sz {
+	// A key matched on a prefix, such as Home for \e[70;5u, or a sequence given
+	// up on partway. rxvt ends keys with the intermediate byte $, as in \e[7$
+	// for SHIFT-HOME, so a parse ending there is complete.
+	if end > *sz && !csiIntermediate(r.buffer[*sz-1]) {
 		*sz = end
 		return Event{Invalid, 0, nil}
 	}

src/tui/light_csi_test.go

@@ -49,10 +49,16 @@ func TestUnknownEscapeSequence(t *testing.T) {
 		{"\x1b[ x", Alt, 2},
 		{"\x1bOx", Alt, 2},
 		{"\x1bO9A", Alt, 2},
-		{"\x1b[2m", Alt, 2}, // the parser gives up on these itself
-		{"\x1b[3x", Alt, 2},
-		{"\x1b[<a", Alt, 2},
-		{"\x1bO<a", Alt, 2},
+
+		// rxvt keys fzf does not know: dropped, not typed
+		{"\x1b[3^", Invalid, 4}, // CTRL-DELETE
+		{"\x1b[5^", Invalid, 4}, // CTRL-PAGEUP
+		{"\x1b[3@", Invalid, 4}, // CTRL-SHIFT-DELETE
+		{"\x1b[11^", Invalid, 5},
+
+		// rxvt ends keys with $, so a key typed after one is not part of it
+		{"\x1b[7$x", Home, 4}, // SHIFT-HOME, then x
+		{"\x1b[3$x", Invalid, 4},
 		{"\x1b]abc", Alt, 2},
 		{"\x1b]11;rgb:", Alt, 2}, // terminator has not arrived
 		{"\x1b]\x1b]", Alt, 2},   // a second sequence must not swallow the ALT key