input: Fix to position cursor at correct offset on mouse down or select. (#580)

Ref: https://github.com/zed-industries/zed/pull/23603

---

## TODO

Here still have a little bug in multiple lines and soft wrapped line.
Only the first line can get correct position, the wrapped line is always
got the offset +1, so the first char of that soft wrapped line, we can't
position the cursor at the first.

crates/ui/src/input/input.rs

@@ -12,7 +12,7 @@ use unicode_segmentation::*;
 use gpui::prelude::FluentBuilder as _;
 use gpui::{
     actions, div, point, px, AnyElement, AppContext, Bounds, ClickEvent, ClipboardItem,
-    Context as _, Entity, EventEmitter, FocusHandle, FocusableView, Half, InteractiveElement as _,
+    Context as _, Entity, EventEmitter, FocusHandle, FocusableView, InteractiveElement as _,
     IntoElement, KeyBinding, KeyDownEvent, Model, MouseButton, MouseDownEvent, MouseMoveEvent,
     MouseUpEvent, ParentElement as _, Pixels, Point, Rems, Render, ScrollHandle, ScrollWheelEvent,
     SharedString, Styled as _, UTF16Selection, ViewContext, ViewInputHandler, WindowContext,
@@ -776,9 +776,9 @@ impl TextInput {
         }
 
         if event.modifiers.shift {
-            self.select_to(self.previous_boundary(offset), cx);
+            self.select_to(offset, cx);
         } else {
-            self.move_to(self.previous_boundary(offset), cx)
+            self.move_to(offset, cx)
         }
     }
 
@@ -896,6 +896,39 @@ impl TextInput {
         }
     }
 
+    // Fork of `closest_index_for_x` method from `gpui::text::Layout`
+    // https://github.com/zed-industries/zed/blob/d1be419fff415329b38f26aff90488700702c82a/crates/gpui/src/text_system/line_layout.rs#L74
+    fn closest_index_for_x(&self, line: &WrappedLine, x: Pixels) -> usize {
+        let mut prev_index = 0;
+        let mut prev_x = px(0.);
+
+        for run in line.unwrapped_layout.runs.iter() {
+            for glyph in run.glyphs.iter() {
+                if glyph.position.x >= x {
+                    if glyph.position.x - x < x - prev_x {
+                        return glyph.index;
+                    } else {
+                        return prev_index;
+                    }
+                }
+                prev_index = glyph.index;
+                prev_x = glyph.position.x;
+            }
+        }
+
+        // HOTFIX:
+        //
+        // Wait https://github.com/zed-industries/zed/pull/23603
+        if line.unwrapped_layout.len == 1 {
+            if x > line.width() / 2.0 {
+                return 1;
+            }
+            return 0;
+        }
+
+        line.unwrapped_layout.len
+    }
+
     fn index_for_mouse_position(&self, position: Point<Pixels>, _: &WindowContext) -> usize {
         // If the text is empty, always return 0
         if self.text.is_empty() {
@@ -926,16 +959,25 @@ impl TextInput {
 
         for line in lines.iter() {
             let line_origin = self.line_origin_with_y_offset(&mut y_offset, &line, line_height);
-            let mut pos = inner_position - line_origin;
-            // Ignore the y position in single line mode, only check x position.
+            let pos = inner_position - line_origin;
+            let closest_index = self.closest_index_for_x(line, pos.x);
+
+            // Return offset by use closest_index_for_x if is single line mode.
             if self.is_single_line() {
-                pos.y = line_height.half();
+                return closest_index;
             }
 
             let index_result = line.index_for_position(pos, line_height);
             if let Ok(v) = index_result {
-                // Add 1 for place cursor after the character.
-                index += v + 1;
+                // FIXME: If the line have soft wrap, only first line can get correct position (closest_index is correct).
+                // The second line or more, the position is always at offset +1,
+                // but if we mouse down on left half or character, we expect the cursor is at offset.
+                // We need find a way to improve.
+                if pos.y > line_height / 2. {
+                    index += v + 1;
+                } else {
+                    index += closest_index;
+                }
                 break;
             } else if let Ok(_) = line.index_for_position(point(px(0.), pos.y), line_height) {
                 // Click in the this line but not in the text, move cursor to the end of the line.
@@ -1168,7 +1210,7 @@ impl TextInput {
         }
 
         let offset = self.index_for_mouse_position(event.position, cx);
-        self.select_to(self.previous_boundary(offset), cx);
+        self.select_to(offset, cx);
     }
 
     fn is_valid_input(&self, new_text: &str) -> bool {