Fix: scope prompt line to its are

This introduces the notion of anchor in the prompt.
The prompt anchor represents the position of the
left-side of the input. This anchor is needed
so that we are able to correctly display the prompt
input when its length is longer than the one of the
prompt area itself

helix-term/src/ui/picker.rs

@@ -649,10 +649,6 @@ impl<T: 'static + Send + Sync, D: 'static + Send + Sync> Picker<T, D> {
 
         // -- Render the input bar:
 
-        let area = inner.clip_left(1).with_height(1);
-        // render the prompt first since it will clear its background
-        self.prompt.render(area, surface, cx);
-
         let count = format!(
             "{}{}/{}",
             if status.running || self.matcher.active_injectors() > 0 {
@@ -663,6 +659,13 @@ impl<T: 'static + Send + Sync, D: 'static + Send + Sync> Picker<T, D> {
             snapshot.matched_item_count(),
             snapshot.item_count(),
         );
+
+        let area = inner.clip_left(1).with_height(1);
+        let line_area = area.clip_right(count.len() as u16 + 1);
+
+        // render the prompt first since it will clear its background
+        self.prompt.render(line_area, surface, cx);
+
         surface.set_stringn(
             (area.x + area.width).saturating_sub(count.len() as u16 + 1),
             area.y,
@@ -1073,7 +1076,15 @@ impl<I: 'static + Send + Sync, D: 'static + Send + Sync> Component for Picker<I,
         let inner = block.inner(area);
 
         // prompt area
-        let area = inner.clip_left(1).with_height(1);
+        let render_preview =
+            self.show_preview && self.file_fn.is_some() && area.width > MIN_AREA_WIDTH_FOR_PREVIEW;
+
+        let picker_width = if render_preview {
+            area.width / 2
+        } else {
+            area.width
+        };
+        let area = inner.clip_left(1).with_height(1).with_width(picker_width);
 
         self.prompt.cursor(area, editor)
     }

helix-term/src/ui/prompt.rs

@@ -28,6 +28,7 @@ pub struct Prompt {
     prompt: Cow<'static, str>,
     line: String,
     cursor: usize,
+    anchor: usize,
     completion: Vec<Completion>,
     selection: Option<usize>,
     history_register: Option<char>,
@@ -80,6 +81,7 @@ impl Prompt {
             prompt,
             line: String::new(),
             cursor: 0,
+            anchor: 0,
             completion: Vec::new(),
             selection: None,
             history_register,
@@ -329,6 +331,7 @@ impl Prompt {
     pub fn clear(&mut self, editor: &Editor) {
         self.line.clear();
         self.cursor = 0;
+
         self.recalculate_completion(editor);
     }
 
@@ -395,13 +398,14 @@ impl Prompt {
 const BASE_WIDTH: u16 = 30;
 
 impl Prompt {
-    pub fn render_prompt(&self, area: Rect, surface: &mut Surface, cx: &mut Context) {
+    pub fn render_prompt(&mut self, area: Rect, surface: &mut Surface, cx: &mut Context) {
         let theme = &cx.editor.theme;
         let prompt_color = theme.get("ui.text");
         let completion_color = theme.get("ui.menu");
         let selected_color = theme.get("ui.menu.selected");
         let suggestion_color = theme.get("ui.text.inactive");
         let background = theme.get("ui.background");
+
         // completion
 
         let max_len = self
@@ -500,7 +504,11 @@ impl Prompt {
         // render buffer text
         surface.set_string(area.x, area.y + line, &self.prompt, prompt_color);
 
-        let line_area = area.clip_left(self.prompt.len() as u16).clip_top(line);
+        let line_area = area
+            .clip_left(self.prompt.len() as u16)
+            .clip_top(line)
+            .clip_right(2);
+
         if self.line.is_empty() {
             // Show the most recently entered value as a suggestion.
             if let Some(suggestion) = self.first_history_completion(cx.editor) {
@@ -517,7 +525,22 @@ impl Prompt {
             .into();
             text.render(line_area, surface, cx);
         } else {
-            surface.set_string(line_area.x, line_area.y, self.line.clone(), prompt_color);
+            if self.line.len() < line_area.width as usize {
+                self.anchor = 0;
+            } else if self.cursor < self.anchor {
+                self.anchor = self.cursor;
+            } else if self.cursor - self.anchor > line_area.width as usize {
+                self.anchor = self.cursor - line_area.width as usize;
+            }
+
+            surface.set_string_anchored(
+                line_area.x,
+                line_area.y,
+                self.anchor,
+                self.line.as_str(),
+                line_area.width as usize,
+                |_| prompt_color,
+            );
         }
     }
 }
@@ -687,14 +710,24 @@ impl Component for Prompt {
     }
 
     fn cursor(&self, area: Rect, editor: &Editor) -> (Option<Position>, CursorKind) {
+        let area = area
+            .clip_left(self.prompt.len() as u16)
+            .clip_right(if self.prompt.len() > 0 { 0 } else { 2 });
+
+        let mut upbound = area.left() as usize
+            + UnicodeWidthStr::width(&self.line[self.anchor..self.cursor.max(self.anchor)]);
+
+        if self.anchor > 0 {
+            upbound += 1;
+        }
+
+        if self.anchor > 0 && self.cursor > self.anchor && self.line.len() > self.cursor {
+            upbound -= 1;
+        }
+
         let line = area.height as usize - 1;
         (
-            Some(Position::new(
-                area.y as usize + line,
-                area.x as usize
-                    + self.prompt.len()
-                    + UnicodeWidthStr::width(&self.line[..self.cursor]),
-            )),
+            Some(Position::new(area.y as usize + line, upbound)),
             editor.config().cursor_shape.from_mode(Mode::Insert),
         )
     }

helix-tui/src/buffer.rs

@@ -306,6 +306,72 @@ impl Buffer {
         self.set_string_truncated_at_end(x, y, string.as_ref(), width, style)
     }
 
+    /// Allow to display a big string in a reduce area.
+    /// A long string will be truncated, with the first character to display starting at the `anchor`.
+    /// The start and end of the string are going to be replaced with an ellipsis (`…`), if needed.
+    #[allow(clippy::too_many_arguments)]
+    pub fn set_string_anchored(
+        &mut self,
+        x: u16,
+        y: u16,
+        anchor: usize,
+        string: &str,
+        width: usize,
+        style: impl Fn(usize) -> Style, // Map a grapheme's string offset to a style
+    ) -> (u16, u16) {
+        // prevent panic if out of range
+        let mut anchor = anchor;
+        if !self.in_bounds(x, y) || width == 0 {
+            return (x, y);
+        }
+
+        let (ellipsis_start, ellipsis_end) = if string.len() <= width {
+            (false, false)
+        } else {
+            (anchor > 0, string.len() - anchor > width)
+        };
+
+        let max_offset = min(
+            self.area.right() as usize - 1,
+            width.saturating_add(x as usize),
+        );
+        let mut start_index = self.index_of(x, y);
+        let mut end_index = self.index_of(max_offset as u16, y);
+
+        if ellipsis_end {
+            self.content[end_index].set_symbol("…");
+            end_index -= 1;
+
+            if ellipsis_start {
+                anchor += 1;
+            }
+        }
+
+        if ellipsis_start {
+            self.content[start_index].set_symbol("…");
+            start_index += 1;
+        }
+
+        let graphemes = string.grapheme_indices(true);
+
+        for (byte_offset, s) in graphemes.skip(anchor) {
+            if start_index > end_index {
+                break;
+            }
+
+            self.content[start_index].set_symbol(s);
+            self.content[start_index].set_style(style(byte_offset));
+
+            for i in start_index + 1..end_index {
+                self.content[i].reset();
+            }
+
+            start_index += s.width();
+        }
+
+        (x, y)
+    }
+
     /// Print at most the first `width` characters of a string if enough space is available
     /// until the end of the line. If `ellipsis` is true appends a `…` at the end of
     /// truncated lines. If `truncate_start` is `true`, truncate the beginning of the string