Avoid erasing OSC 133 prompt start marker with clr_eol

For multi-line prompts, we start each leading line with a clr_eol.  Immediately
before printing these prompt lines we emit the OSC 133 prompt start marker.
Some terminals such as tmux interpret make clr_eol delete such markers,
hence prompt navigation is broken.

Fix this by printing the marker only after clr_eol.

The scenario where this triggers is quite odd.  I haven't looked into why
the problem doesn't exist if I remove the recursive repaint request.

See https://github.com/tmux/tmux/issues/4183
Closes #10776

src/screen.rs

@@ -821,12 +821,19 @@ impl Screen {
         // Output the left prompt if it has changed.
         if left_prompt != zelf.actual_left_prompt {
             zelf.r#move(0, 0);
-            zelf.write_bytes(b"\x1b]133;A;special_key=1\x07");
             let mut start = 0;
-            for line_break in left_prompt_layout.line_breaks {
+            let osc_133_prompt_start =
+                |zelf: &mut Screen| zelf.write_bytes(b"\x1b]133;A;special_key=1\x07");
+            if left_prompt_layout.line_breaks.is_empty() {
+                osc_133_prompt_start(&mut zelf);
+            }
+            for (i, &line_break) in left_prompt_layout.line_breaks.iter().enumerate() {
                 zelf.outp
                     .borrow_mut()
                     .tputs_if_some(&term.and_then(|term| term.clr_eol.as_ref()));
+                if i == 0 {
+                    osc_133_prompt_start(&mut zelf);
+                }
                 zelf.write_str(&left_prompt[start..=line_break]);
                 start = line_break + 1;
             }

tests/checks/tmux-signal-multiline-prompt.fish

@@ -0,0 +1,34 @@
+#RUN: %fish %s
+#REQUIRES: command -v tmux
+
+isolated-tmux-start
+
+isolated-tmux send-keys '
+    function fish_prompt
+        printf "prompt-line-1\\nprompt-line-2> "
+        commandline -f repaint
+    end
+' Enter
+isolated-tmux send-keys C-l \
+    ': 1' Enter \
+    ': 3' Enter \
+    ': 5' Enter
+tmux-sleep
+
+# Screen looks like
+
+# [y=0] prompt-line-1
+# [y=1] prompt-line-2> : 1
+# [y=2] prompt-line-1
+# [y=3] prompt-line-2> : 3
+# [y=4] prompt-line-1
+# [y=5] prompt-line-2> : 5
+# [y=6] prompt-line-1
+# [y=7] prompt-line-2>
+
+isolated-tmux copy-mode
+isolated-tmux send-keys -X previous-prompt
+isolated-tmux send-keys -X previous-prompt
+tmux-sleep
+isolated-tmux display-message -p '#{copy_cursor_y} #{copy_cursor_line}'
+# CHECK: 4 prompt-line-1