Implement `ansi::ClearMode::Saved`

The clearing the screen for the `ansi::ClearMode::Saved` enum value
has been implemented. This is used to clear all lines which are
currently outside of the visible region but still inside the scrollback
buffer.

The specifications of XTerm indicate that the clearing of saved lines
should only clear the saved lines and not the saved lines plus the
currently visible part of the grid. Applications like `clear` send both
the escape for clearing history plus the escape for clearing history
when requested, so all sources seem to agree here.

To allow both clearing the screen and the saved lines when a key is
pressed the `process_key_bindings` method has been altered so multiple
bindings can be specified. So it is now possible to execute both `^L`
and `ClearHistory` with just a single binding. The
`process_mouse_bindings` method has also been changed for consistency.

To make sure everything works properly a test has been added which
clears the history and then attempts to scroll. Since scrolling is the
only way for a user to check if scrollback is available, this seems like
a nice abstraction to check if there is a scrollback.

alacritty_macos.yml

@@ -315,6 +315,8 @@ key_bindings:
   - { key: Key0,     mods: Command, action: ResetFontSize                }
   - { key: Equals,   mods: Command, action: IncreaseFontSize             }
   - { key: Minus,    mods: Command, action: DecreaseFontSize             }
+  - { key: K,        mods: Command, action: ClearHistory                 }
+  - { key: K,        mods: Command, chars: "\x0c"                        }
   - { key: PageUp,   mods: Shift,   chars: "\x1b[5;2~"                   }
   - { key: PageUp,   mods: Control, chars: "\x1b[5;5~"                   }
   - { key: PageUp,                  chars: "\x1b[5~"                     }

src/config.rs

@@ -598,7 +598,7 @@ impl<'a> de::Deserialize<'a> for ActionWrapper {
             fn expecting(&self, f: &mut fmt::Formatter) -> fmt::Result {
                 f.write_str("Paste, Copy, PasteSelection, IncreaseFontSize, DecreaseFontSize, \
                             ResetFontSize, ScrollPageUp, ScrollPageDown, ScrollToTop, \
-                            ScrollToBottom, Hide, or Quit")
+                            ScrollToBottom, ClearHistory, Hide, or Quit")
             }
 
             fn visit_str<E>(self, value: &str) -> ::std::result::Result<ActionWrapper, E>
@@ -615,6 +615,7 @@ impl<'a> de::Deserialize<'a> for ActionWrapper {
                     "ScrollPageDown" => Action::ScrollPageDown,
                     "ScrollToTop" => Action::ScrollToTop,
                     "ScrollToBottom" => Action::ScrollToBottom,
+                    "ClearHistory" => Action::ClearHistory,
                     "Hide" => Action::Hide,
                     "Quit" => Action::Quit,
                     _ => return Err(E::invalid_value(Unexpected::Str(value), &self)),

src/event.rs

@@ -10,6 +10,7 @@ use parking_lot::MutexGuard;
 use glutin::{self, ModifiersState, Event, ElementState};
 use copypasta::{Clipboard, Load, Store};
 
+use ansi::{Handler, ClearMode};
 use grid::Scroll;
 use config::{self, Config};
 use cli::Options;
@@ -59,6 +60,10 @@ impl<'a, N: Notify + 'a> input::ActionContext for ActionContext<'a, N> {
         self.terminal.scroll_display(scroll);
     }
 
+    fn clear_history(&mut self) {
+        self.terminal.clear_screen(ClearMode::Saved);
+    }
+
     fn copy_selection(&self, buffer: ::copypasta::Buffer) {
         if let Some(selected) = self.terminal.selection_to_string() {
             if !selected.is_empty() {

src/grid/mod.rs

@@ -219,14 +219,9 @@ impl<T: Copy + Clone> Grid<T> {
 
     /// Add lines to the visible area
     ///
-    /// The behavior in Terminal.app and iTerm.app is to keep the cursor at the
-    /// bottom of the screen as long as there is scrollback available. Once
-    /// scrollback is exhausted, new lines are simply added to the bottom of the
-    /// screen.
-    ///
-    /// Alacritty takes a different approach. Rather than trying to move with
-    /// the scrollback, we simply pull additional lines from the back of the
-    /// buffer in order to populate the new area.
+    /// Alacritty keeps the cursor at the bottom of the terminal as long as there
+    /// is scrollback available. Once scrollback is exhausted, new lines are
+    /// simply added to the bottom of the screen.
     fn grow_lines(
         &mut self,
         new_line_count: index::Line,

src/input.rs

@@ -69,6 +69,7 @@ pub trait ActionContext {
     fn change_font_size(&mut self, delta: f32);
     fn reset_font_size(&mut self);
     fn scroll(&mut self, scroll: Scroll);
+    fn clear_history(&mut self);
     fn hide_window(&mut self);
 }
 
@@ -183,6 +184,9 @@ pub enum Action {
     /// Scroll all the way to the bottom
     ScrollToBottom,
 
+    /// Clear the display buffer(s) to remove history
+    ClearHistory,
+
     /// Run given command
     Command(String, Vec<String>),
 
@@ -272,6 +276,9 @@ impl Action {
             Action::ScrollToBottom => {
                 ctx.scroll(Scroll::Bottom);
             },
+            Action::ClearHistory => {
+                ctx.clear_history();
+            },
         }
     }
 
@@ -634,15 +641,16 @@ impl<'a, A: ActionContext + 'a> Processor<'a, A> {
     ///
     /// Returns true if an action is executed.
     fn process_key_bindings(&mut self, mods: ModifiersState, key: VirtualKeyCode) -> bool {
+        let mut has_binding = false;
         for binding in self.key_bindings {
             if binding.is_triggered_by(self.ctx.terminal_mode(), mods, &key) {
                 // binding was triggered; run the action
                 binding.execute(&mut self.ctx);
-                return true;
+                has_binding = true;
             }
         }
 
-        false
+        has_binding
     }
 
     /// Attempts to find a binding and execute its action
@@ -652,15 +660,16 @@ impl<'a, A: ActionContext + 'a> Processor<'a, A> {
     ///
     /// Returns true if an action is executed.
     fn process_mouse_bindings(&mut self, mods: ModifiersState, button: MouseButton) -> bool {
+        let mut has_binding = false;
         for binding in self.mouse_bindings {
             if binding.is_triggered_by(self.ctx.terminal_mode(), mods, &button) {
                 // binding was triggered; run the action
                 binding.execute(&mut self.ctx);
-                return true;
+                has_binding = true;
             }
         }
 
-        false
+        has_binding
     }
 }
 
@@ -756,6 +765,8 @@ mod tests {
         }
         fn reset_font_size(&mut self) {
         }
+        fn clear_history(&mut self) {
+        }
         fn hide_window(&mut self) {
         }
     }

src/term/mod.rs

@@ -1797,7 +1797,9 @@ impl ansi::Handler for Term {
                 }
             },
             // If scrollback is implemented, this should clear it
-            ansi::ClearMode::Saved => return
+            ansi::ClearMode::Saved => {
+                self.grid.clear_history();
+            }
         }
     }
 
@@ -2006,9 +2008,9 @@ mod tests {
     use super::{Cell, Term, SizeInfo};
     use term::cell;
 
-    use grid::Grid;
+    use grid::{Grid, Scroll};
     use index::{Point, Line, Column};
-    use ansi::{Handler, CharsetIndex, StandardCharset};
+    use ansi::{self, Handler, CharsetIndex, StandardCharset};
     use selection::Selection;
     use std::mem;
     use input::FONT_SIZE_STEP;
@@ -2183,6 +2185,31 @@ mod tests {
         let expected_font_size: Size = config.font().size();
         assert_eq!(term.font_size, expected_font_size);
     }
+
+    #[test]
+    fn clear_saved_lines() {
+        let size = SizeInfo {
+            width: 21.0,
+            height: 51.0,
+            cell_width: 3.0,
+            cell_height: 3.0,
+            padding_x: 0.0,
+            padding_y: 0.0,
+        };
+        let config: Config = Default::default();
+        let mut term: Term = Term::new(&config, size);
+
+        // Add one line of scrollback
+        term.grid.scroll_up(&(Line(0)..Line(1)), Line(1), &Cell::default());
+
+        // Clear the history
+        term.clear_screen(ansi::ClearMode::Saved);
+
+        // Make sure that scrolling does not change the grid
+        let mut scrolled_grid = term.grid.clone();
+        scrolled_grid.scroll_display(Scroll::Top);
+        assert_eq!(term.grid, scrolled_grid);
+    }
 }
 
 #[cfg(all(test, feature = "bench"))]