Implement "has_leading_newline"

This change adds a new TermOption, "has_leading_newline".

When this option is set to false (default is true),
TermLock::ensure_height() does not add a leading newline
(=the separator between the original line contents and the new
contents) to the output anymore.

When it is set to true, there is no change in ensure_height()'s
behavior (the leading newline is present).

src/term.rs

@@ -73,6 +73,7 @@ pub struct TermOptions {
     mouse_enabled: bool,
     raw_mouse: bool,
     hold: bool, // to start term or not on creation
+    has_leading_newline: bool,
 }
 
 impl Default for TermOptions {
@@ -85,6 +86,7 @@ impl Default for TermOptions {
             mouse_enabled: false,
             raw_mouse: false,
             hold: false,
+            has_leading_newline: true,
         }
     }
 }
@@ -120,6 +122,10 @@ impl TermOptions {
         self.hold = hold;
         self
     }
+    pub fn has_leading_newline(mut self, has_leading_newline: bool) -> Self {
+        self.has_leading_newline = has_leading_newline;
+        self
+    }
 }
 
 impl<UserEvent: Send + 'static> Term<UserEvent> {
@@ -563,6 +569,7 @@ struct TermLock {
     bottom_intact: bool,
     clear_on_exit: bool,
     mouse_enabled: bool,
+    has_leading_newline: bool,
     alternate_screen: bool,
     cursor_row: usize,
     screen_height: usize,
@@ -578,6 +585,7 @@ impl Default for TermLock {
             max_height: TermHeight::Percent(100),
             min_height: TermHeight::Fixed(3),
             bottom_intact: false,
+            has_leading_newline: true,
             alternate_screen: false,
             cursor_row: 0,
             screen_height: 0,
@@ -598,6 +606,7 @@ impl TermLock {
         term.min_height = options.min_height;
         term.clear_on_exit = options.clear_on_exit;
         term.mouse_enabled = options.mouse_enabled;
+        term.has_leading_newline = options.has_leading_newline;
         term
     }
 
@@ -744,8 +753,7 @@ impl TermLock {
         } else {
             // only use part of the screen
 
-            // go to a new line so that existing line won't be messed up
-            if cursor_col > 0 {
+            if self.has_leading_newline && cursor_col > 0 {
                 output.write("\n");
                 cursor_row += 1;
             }