Fix empty block selection detection

Fixes #2789.

alacritty_terminal/src/selection.rs

@@ -132,7 +132,7 @@ impl Selection {
 
     pub fn is_empty(&self) -> bool {
         match *self {
-            Selection::Simple { ref region } | Selection::Block { ref region } => {
+            Selection::Simple { ref region } => {
                 let (start, end) =
                     if Selection::points_need_swap(region.start.point, region.end.point) {
                         (&region.end, &region.start)
@@ -140,13 +140,26 @@ impl Selection {
                         (&region.start, &region.end)
                     };
 
-                // Empty when single cell with identical sides or two cell with right+left sides
+                // Simple selection is empty when the points are identical
+                // or two adjacent cells have the sides right -> left
                 start == end
                     || (start.side == Side::Left
                         && end.side == Side::Right
-                        && start.point.line == end.point.line
+                        && (start.point.line == end.point.line)
                         && start.point.col == end.point.col + 1)
             },
+            Selection::Block { region: Range { ref start, ref end } } => {
+                // Block selection is empty when the points' columns and sides are identical
+                // or two cells with adjacent columns have the sides right -> left,
+                // regardless of their lines
+                (start.point.col == end.point.col && start.side == end.side)
+                    || (start.point.col == end.point.col + 1
+                        && start.side == Side::Left
+                        && end.side == Side::Right)
+                    || (end.point.col == start.point.col + 1
+                        && end.side == Side::Left
+                        && start.side == Side::Right)
+            },
             Selection::Semantic { .. } | Selection::Lines { .. } => false,
         }
     }
@@ -589,4 +602,30 @@ mod test {
             is_block: false,
         });
     }
+
+    #[test]
+    fn simple_is_empty() {
+        let mut selection = Selection::simple(Point::new(0, Column(0)), Side::Right);
+        assert!(selection.is_empty());
+        selection.update(Point::new(0, Column(1)), Side::Left);
+        assert!(selection.is_empty());
+        selection.update(Point::new(1, Column(0)), Side::Right);
+        assert!(!selection.is_empty());
+    }
+
+    #[test]
+    fn block_is_empty() {
+        let mut selection = Selection::block(Point::new(0, Column(0)), Side::Right);
+        assert!(selection.is_empty());
+        selection.update(Point::new(0, Column(1)), Side::Left);
+        assert!(selection.is_empty());
+        selection.update(Point::new(0, Column(1)), Side::Right);
+        assert!(!selection.is_empty());
+        selection.update(Point::new(1, Column(0)), Side::Right);
+        assert!(selection.is_empty());
+        selection.update(Point::new(1, Column(1)), Side::Left);
+        assert!(selection.is_empty());
+        selection.update(Point::new(1, Column(1)), Side::Right);
+        assert!(!selection.is_empty());
+    }
 }

alacritty_terminal/src/tty/unix.rs

@@ -28,6 +28,7 @@ use mio::unix::EventedFd;
 use std::ffi::CStr;
 use std::fs::File;
 use std::io;
+use std::mem::MaybeUninit;
 use std::os::unix::{
     io::{AsRawFd, FromRawFd, RawFd},
     process::CommandExt,
@@ -35,7 +36,6 @@ use std::os::unix::{
 use std::process::{Child, Command, Stdio};
 use std::ptr;
 use std::sync::atomic::{AtomicUsize, Ordering};
-use std::mem::MaybeUninit;
 
 /// Process ID of child process
 ///