Fix spurious blank lines when executing scrolled commandline

The result of

	commandline -i ": '$(seq $LINES)"\n"first scrolled line'"

is a commandline that is scrolled by one line.

Before executing that commandline, we move the cursor down by one
too many line. This is a regression from 610338cc70 (On undo after
execute, restore the cursor position, 2024-12-21). Fix that.

The test also demonstrates an unrelated problem, probably specific
to tmux.

src/reader.rs

@@ -2134,7 +2134,9 @@ fn readline(&mut self, nchars: Option<NonZeroUsize>) -> Option<WString> {
 
         // Redraw the command line. This is what ensures the autosuggestion is hidden, etc. after the
         // user presses enter.
-        if zelf.is_repaint_needed(None) || zelf.screen.scrolled || zelf.conf.inputfd != STDIN_FILENO
+        if zelf.is_repaint_needed(None)
+            || zelf.screen.scrolled()
+            || zelf.conf.inputfd != STDIN_FILENO
         {
             zelf.layout_and_repaint_before_execution();
         }

src/screen.rs

@@ -189,8 +189,9 @@ pub fn is_empty(&self) -> bool {
 pub struct Screen {
     /// Whether the last-drawn autosuggestion (if any) is truncated, or hidden entirely.
     pub autosuggestion_is_truncated: bool,
-    /// True if the last rendering was so large we could only display part of the command line.
-    pub scrolled: bool,
+    /// If the last rendering was so large we could only display part of the command line,
+    /// this is the number of lines that were pushed to scrollback.
+    pub scroll_amount: usize,
 
     /// Receiver for our output.
     outp: &'static RefCell<Outputter>,
@@ -227,7 +228,7 @@ pub fn new() -> Self {
         Self {
             outp: Outputter::stdoutput(),
             autosuggestion_is_truncated: Default::default(),
-            scrolled: Default::default(),
+            scroll_amount: Default::default(),
             desired: Default::default(),
             actual: Default::default(),
             actual_left_prompt: Default::default(),
@@ -241,6 +242,10 @@ pub fn new() -> Self {
         }
     }
 
+    pub fn scrolled(&self) -> bool {
+        self.scroll_amount != 0
+    }
+
     /// This is the main function for the screen output library. It is used to define the desired
     /// contents of the screen. The screen command will use its knowledge of the current contents of
     /// the screen in order to render the desired output using as few terminal commands as possible.
@@ -459,7 +464,7 @@ struct ScrolledCursor {
         // Append pager_data (none if empty).
         self.desired.append_lines(&page_rendering.screen_data);
 
-        self.scrolled = scrolled_cursor.scroll_amount != 0;
+        self.scroll_amount = scrolled_cursor.scroll_amount;
 
         self.update(
             vars,
@@ -500,7 +505,7 @@ pub fn reset_line(&mut self, repaint_prompt: bool /* = false */) {
     }
 
     pub fn move_to_end(&mut self) {
-        self.r#move(0, self.actual.line_count());
+        self.r#move(0, self.actual.line_count() - self.scroll_amount);
     }
 
     pub fn push_to_scrollback(&mut self, cursor_y: usize) {
@@ -1035,14 +1040,15 @@ fn update(
         let term = term.as_ref();
 
         // Output the left prompt if it has changed.
-        if zelf.scrolled && !is_final_rendering {
+        if zelf.scrolled() && !is_final_rendering {
             zelf.r#move(0, 0);
             zelf.outp
                 .borrow_mut()
                 .tputs_if_some(&term.and_then(|term| term.clr_eol.as_ref()));
             zelf.actual_left_prompt.clear();
             zelf.actual.cursor.x = 0;
-        } else if left_prompt != zelf.actual_left_prompt || (zelf.scrolled && is_final_rendering) {
+        } else if left_prompt != zelf.actual_left_prompt || (zelf.scrolled() && is_final_rendering)
+        {
             zelf.r#move(0, 0);
             let mut start = 0;
             let osc_133_prompt_start =
@@ -1092,7 +1098,7 @@ fn s_line(zelf: &Screen, i: usize) -> &Line {
             // Note that skip_remaining is a width, not a character count.
             let mut skip_remaining = start_pos;
 
-            let shared_prefix = if zelf.scrolled {
+            let shared_prefix = if zelf.scrolled() {
                 0
             } else {
                 line_shared_prefix(o_line(&zelf, i), s_line(&zelf, i))

tests/checks/tmux-commandline.fish

@@ -32,6 +32,23 @@ isolated-tmux capture-pane -p
 # CHECK: 10
 # CHECK: scroll_here
 
+# The output is broken here (seems tmux specific).
+isolated-tmux send-keys C-c
+tmux-sleep
+isolated-tmux send-keys C-l 'commandline -i ": \'$(seq $LINES)" A B "C\'"' Enter Enter
+tmux-sleep
+isolated-tmux capture-pane -p
+# CHECK: 4
+# CHECK: 5
+# CHECK: 6
+# CHECK: 7
+# CHECK: 8
+# CHECK: 9
+# CHECK: 10
+# CHECK: prompt 5>
+# CHECK: B
+# CHECK: C'
+
 # Soft-wrapped commandline with omitted right prompt.
 isolated-tmux send-keys C-c
 tmux-sleep
@@ -43,7 +60,7 @@ isolated-tmux send-keys C-l '
 ' Enter
 tmux-sleep
 isolated-tmux capture-pane -p | sed 1,5d
-# CHECK: prompt 5> echo 00000000000000000000000000000000000000000000000000000000000000000
+# CHECK: prompt {{\d+}}> echo 00000000000000000000000000000000000000000000000000000000000000000
 # CHECK: 000000000000000
 # CHECK: 00000000000000000000000000000000000000000000000000000000000000000000000000000000
-# CHECK: prompt 6>                                                           right-prompt
+# CHECK: prompt {{\d+}}>                                                           right-prompt