Fix selection start point lagging behind cursor

Since the mouse start position has been the first movement event after
the mouse button was held down, there have been some issues with the
start point lagging behind the cursor because movement events were not
reported from the initial position but there was a gap until movement
starts reporting.

To fix this whenever the mouse button is pressed, the position and cell
side is stored on the `Mouse` struct. Because of this it does not matter
anymore if the movement events are all reported and we can just start a
selection using the stored position/side whenever there currently is no
selection present.

This fixes #1366

2 files changed, 54 insertions, 32 deletions

diff --git a/src/event.rs b/src/event.rs
index da63d5fd..9db0680d 100644
--- a/src/event.rs
+++ b/src/event.rs
@@ -82,17 +82,14 @@ impl<'a, N: Notify + 'a> input::ActionContext for ActionContext<'a, N> {
     }
 
     fn update_selection(&mut self, point: Point, side: Side) {
-        self.terminal.dirty = true;
         let point = self.terminal.visible_to_buffer(point);
 
         // Update selection if one exists
-        if let Some(ref mut selection) = *self.terminal.selection_mut() {
+        if let Some(ref mut selection) = self.terminal.selection_mut() {
             selection.update(point, side);
-            return;
         }
 
-        // Otherwise, start a regular selection
-        *self.terminal.selection_mut() = Some(Selection::simple(point, side));
+        self.terminal.dirty = true;
     }
 
     fn simple_selection(&mut self, point: Point, side: Side) {
@@ -131,6 +128,11 @@ impl<'a, N: Notify + 'a> input::ActionContext for ActionContext<'a, N> {
     }
 
     #[inline]
+    fn mouse(&self) -> &Mouse {
+        self.mouse
+    }
+
+    #[inline]
     fn received_count(&mut self) -> &mut usize {
         &mut self.received_count
     }
diff --git a/src/input.rs b/src/input.rs
index 59b6605e..cc3f13df 100644
--- a/src/input.rs
+++ b/src/input.rs
@@ -63,6 +63,7 @@ pub trait ActionContext {
     fn semantic_selection(&mut self, point: Point);
     fn line_selection(&mut self, point: Point);
     fn mouse_mut(&mut self) -> &mut Mouse;
+    fn mouse(&self) -> &Mouse;
     fn mouse_coords(&self) -> Option<Point>;
     fn received_count(&mut self) -> &mut usize;
     fn suppress_chars(&mut self) -> &mut bool;
@@ -318,41 +319,27 @@ impl<'a, A: ActionContext + 'a> Processor<'a, A> {
         let prev_line = mem::replace(&mut self.ctx.mouse_mut().line, point.line);
         let prev_col = mem::replace(&mut self.ctx.mouse_mut().column, point.col);
 
-        let cell_x = x.saturating_sub(size_info.padding_x as usize) % size_info.cell_width as usize;
-        let half_cell_width = (size_info.cell_width / 2.0) as usize;
-
-        let additional_padding = (size_info.width - size_info.padding_x * 2.) % size_info.cell_width;
-        let end_of_grid = size_info.width - size_info.padding_x - additional_padding;
-        let cell_side = if cell_x > half_cell_width
-            // Edge case when mouse leaves the window
-            || x as f32 >= end_of_grid
-        {
-            Side::Right
-        } else {
-            Side::Left
-        };
-        self.ctx.mouse_mut().cell_side = cell_side;
-
         let motion_mode = TermMode::MOUSE_MOTION | TermMode::MOUSE_DRAG;
         let report_mode = TermMode::MOUSE_REPORT_CLICK | motion_mode;
 
-        if self.ctx.mouse_mut().left_button_state == ElementState::Pressed &&
+        if self.ctx.mouse().left_button_state == ElementState::Pressed &&
             ( modifiers.shift || !self.ctx.terminal_mode().intersects(report_mode))
         {
+            let cell_side = self.get_mouse_side();
             self.ctx.update_selection(Point {
                 line: point.line,
                 col: point.col
             }, cell_side);
         } else if self.ctx.terminal_mode().intersects(motion_mode)
             // Only report motion when changing cells
-            && (prev_line != self.ctx.mouse_mut().line || prev_col != self.ctx.mouse_mut().column)
+            && (prev_line != self.ctx.mouse().line || prev_col != self.ctx.mouse().column)
             && size_info.contains_point(x, y)
         {
-            if self.ctx.mouse_mut().left_button_state == ElementState::Pressed {
+            if self.ctx.mouse().left_button_state == ElementState::Pressed {
                 self.mouse_report(32, ElementState::Pressed, modifiers);
-            } else if self.ctx.mouse_mut().middle_button_state == ElementState::Pressed {
+            } else if self.ctx.mouse().middle_button_state == ElementState::Pressed {
                 self.mouse_report(33, ElementState::Pressed, modifiers);
-            } else if self.ctx.mouse_mut().right_button_state == ElementState::Pressed {
+            } else if self.ctx.mouse().right_button_state == ElementState::Pressed {
                 self.mouse_report(34, ElementState::Pressed, modifiers);
             } else if self.ctx.terminal_mode().contains(TermMode::MOUSE_MOTION) {
                 self.mouse_report(35, ElementState::Pressed, modifiers);
@@ -361,7 +348,7 @@ impl<'a, A: ActionContext + 'a> Processor<'a, A> {
     }
 
     pub fn normal_mouse_report(&mut self, button: u8) {
-        let (line, column) = (self.ctx.mouse_mut().line, self.ctx.mouse_mut().column);
+        let (line, column) = (self.ctx.mouse().line, self.ctx.mouse().column);
 
         if line < Line(223) && column < Column(223) {
             let msg = vec![
@@ -378,7 +365,7 @@ impl<'a, A: ActionContext + 'a> Processor<'a, A> {
     }
 
     pub fn sgr_mouse_report(&mut self, button: u8, state: ElementState) {
-        let (line, column) = (self.ctx.mouse_mut().line, self.ctx.mouse_mut().column);
+        let (line, column) = (self.ctx.mouse().line, self.ctx.mouse().column);
         let c = match state {
             ElementState::Pressed => 'M',
             ElementState::Released => 'm',
@@ -425,10 +412,10 @@ impl<'a, A: ActionContext + 'a> Processor<'a, A> {
 
     pub fn on_mouse_press(&mut self, button: MouseButton, modifiers: ModifiersState) {
         let now = Instant::now();
-        let elapsed = self.ctx.mouse_mut().last_click_timestamp.elapsed();
+        let elapsed = self.ctx.mouse().last_click_timestamp.elapsed();
         self.ctx.mouse_mut().last_click_timestamp = now;
 
-        self.ctx.mouse_mut().click_state = match self.ctx.mouse_mut().click_state {
+        self.ctx.mouse_mut().click_state = match self.ctx.mouse().click_state {
             ClickState::Click if elapsed < self.mouse_config.double_click.threshold => {
                 self.on_mouse_double_click();
                 ClickState::DoubleClick
@@ -439,6 +426,13 @@ impl<'a, A: ActionContext + 'a> Processor<'a, A> {
             },
             _ => {
                 self.ctx.clear_selection();
+
+                // Start new empty selection
+                if let Some(point) = self.ctx.mouse_coords() {
+                    let side = self.get_mouse_side();
+                    self.ctx.simple_selection(point, side);
+                }
+
                 let report_modes = TermMode::MOUSE_REPORT_CLICK | TermMode::MOUSE_DRAG | TermMode::MOUSE_MOTION;
                 if !modifiers.shift && self.ctx.terminal_mode().intersects(report_modes) {
                     match button {
@@ -456,6 +450,26 @@ impl<'a, A: ActionContext + 'a> Processor<'a, A> {
         };
     }
 
+    fn get_mouse_side(&self) -> Side {
+        let size_info = self.ctx.size_info();
+        let x = self.ctx.mouse().x;
+
+        let cell_x = x.saturating_sub(size_info.padding_x as usize) % size_info.cell_width as usize;
+        let half_cell_width = (size_info.cell_width / 2.0) as usize;
+
+        let additional_padding = (size_info.width - size_info.padding_x * 2.) % size_info.cell_width;
+        let end_of_grid = size_info.width - size_info.padding_x - additional_padding;
+
+        if cell_x > half_cell_width
+            // Edge case when mouse leaves the window
+            || x as f32 >= end_of_grid
+        {
+            Side::Right
+        } else {
+            Side::Left
+        }
+    }
+
     pub fn on_mouse_release(&mut self, button: MouseButton, modifiers: ModifiersState) {
         let report_modes = TermMode::MOUSE_REPORT_CLICK | TermMode::MOUSE_DRAG | TermMode::MOUSE_MOTION;
         if !modifiers.shift && self.ctx.terminal_mode().intersects(report_modes)
@@ -479,7 +493,7 @@ impl<'a, A: ActionContext + 'a> Processor<'a, A> {
     pub fn on_mouse_wheel(&mut self, delta: MouseScrollDelta, phase: TouchPhase, modifiers: ModifiersState) {
         match delta {
             MouseScrollDelta::LineDelta(_columns, lines) => {
-                let to_scroll = self.ctx.mouse_mut().lines_scrolled + lines;
+                let to_scroll = self.ctx.mouse().lines_scrolled + lines;
                 let code = if to_scroll > 0.0 {
                     64
                 } else {
@@ -503,8 +517,8 @@ impl<'a, A: ActionContext + 'a> Processor<'a, A> {
                         self.ctx.mouse_mut().scroll_px += y as i32;
                         let height = self.ctx.size_info().cell_height as i32;
 
-                        while self.ctx.mouse_mut().scroll_px.abs() >= height {
-                            let code = if self.ctx.mouse_mut().scroll_px > 0 {
+                        while self.ctx.mouse().scroll_px.abs() >= height {
+                            let code = if self.ctx.mouse().scroll_px > 0 {
                                 self.ctx.mouse_mut().scroll_px -= height;
                                 64
                             } else {
@@ -757,6 +771,12 @@ mod tests {
         fn mouse_mut(&mut self) -> &mut Mouse {
             self.mouse
         }
+
+        #[inline]
+        fn mouse(&self) -> &Mouse {
+            self.mouse
+        }
+
         fn received_count(&mut self) -> &mut usize {
             &mut self.received_count
         }