Fix: dragging to above/below a `TextEdit` or `Label` will select text to begin/end (#3858)

This also adds a `Galley::begin` method to match `Galley::end` (both
returning cursor positions).

* Part of https://github.com/emilk/egui/issues/3816

crates/egui/src/text_selection/cursor_range.rs

@@ -38,7 +38,7 @@ impl CursorRange {
 
     /// Select all the text in a galley
     pub fn select_all(galley: &Galley) -> Self {
-        Self::two(Cursor::default(), galley.end())
+        Self::two(galley.begin(), galley.end())
     }
 
     pub fn as_ccursor_range(&self) -> CCursorRange {
@@ -342,7 +342,7 @@ fn move_single_cursor(
         Key::ArrowUp => {
             if modifiers.command {
                 // mac and windows behavior
-                *cursor = Cursor::default();
+                *cursor = galley.begin();
             } else {
                 *cursor = galley.cursor_up_one_row(cursor);
             }
@@ -359,7 +359,7 @@ fn move_single_cursor(
         Key::Home => {
             if modifiers.ctrl {
                 // windows behavior
-                *cursor = Cursor::default();
+                *cursor = galley.begin();
             } else {
                 *cursor = galley.cursor_begin_of_row(cursor);
             }

crates/egui/src/text_selection/text_cursor_state.rs

@@ -20,6 +20,27 @@ pub struct TextCursorState {
     ccursor_range: Option<CCursorRange>,
 }
 
+impl From<CursorRange> for TextCursorState {
+    fn from(cursor_range: CursorRange) -> Self {
+        Self {
+            cursor_range: Some(cursor_range),
+            ccursor_range: Some(CCursorRange {
+                primary: cursor_range.primary.ccursor,
+                secondary: cursor_range.secondary.ccursor,
+            }),
+        }
+    }
+}
+
+impl From<CCursorRange> for TextCursorState {
+    fn from(ccursor_range: CCursorRange) -> Self {
+        Self {
+            cursor_range: None,
+            ccursor_range: Some(ccursor_range),
+        }
+    }
+}
+
 impl TextCursorState {
     pub fn is_empty(&self) -> bool {
         self.cursor_range.is_none() && self.ccursor_range.is_none()
@@ -33,7 +54,7 @@ impl TextCursorState {
         })
     }
 
-    pub fn range(&mut self, galley: &Galley) -> Option<CursorRange> {
+    pub fn range(&self, galley: &Galley) -> Option<CursorRange> {
         self.cursor_range
             .map(|cursor_range| {
                 // We only use the PCursor (paragraph number, and character offset within that paragraph).

crates/epaint/src/text/cursor.rs

@@ -1,6 +1,8 @@
 //! Different types of text cursors, i.e. ways to point into a [`super::Galley`].
 
-/// Character cursor
+/// Character cursor.
+///
+/// The default cursor is zero.
 #[derive(Clone, Copy, Debug, Default)]
 #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct CCursor {
@@ -110,9 +112,12 @@ impl PartialEq for PCursor {
 }
 
 /// All different types of cursors together.
+///
 /// They all point to the same place, but in their own different ways.
 /// pcursor/rcursor can also point to after the end of the paragraph/row.
 /// Does not implement `PartialEq` because you must think which cursor should be equivalent.
+///
+/// The default cursor is the zero-cursor, to the first character.
 #[derive(Clone, Copy, Debug, Default, PartialEq)]
 #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
 pub struct Cursor {

crates/epaint/src/text/text_layout_types.rs

@@ -712,8 +712,25 @@ impl Galley {
         self.pos_from_pcursor(cursor.pcursor) // pcursor is what TextEdit stores
     }
 
-    /// Cursor at the given position within the galley
+    /// Cursor at the given position within the galley.
+    ///
+    /// A cursor above the galley is considered
+    /// same as a cursor at the start,
+    /// and a cursor below the galley is considered
+    /// same as a cursor at the end.
+    /// This allows implementing text-selection by dragging above/below the galley.
     pub fn cursor_from_pos(&self, pos: Vec2) -> Cursor {
+        if let Some(first_row) = self.rows.first() {
+            if pos.y < first_row.min_y() {
+                return self.begin();
+            }
+        }
+        if let Some(last_row) = self.rows.last() {
+            if last_row.max_y() < pos.y {
+                return self.end();
+            }
+        }
+
         let mut best_y_dist = f32::INFINITY;
         let mut cursor = Cursor::default();
 
@@ -721,7 +738,7 @@ impl Galley {
         let mut pcursor_it = PCursor::default();
 
         for (row_nr, row) in self.rows.iter().enumerate() {
-            let is_pos_within_row = pos.y >= row.min_y() && pos.y <= row.max_y();
+            let is_pos_within_row = row.min_y() <= pos.y && pos.y <= row.max_y();
             let y_dist = (row.min_y() - pos.y).abs().min((row.max_y() - pos.y).abs());
             if is_pos_within_row || y_dist < best_y_dist {
                 best_y_dist = y_dist;
@@ -755,12 +772,22 @@ impl Galley {
                 pcursor_it.offset += row.char_count_including_newline();
             }
         }
+
         cursor
     }
 }
 
 /// ## Cursor positions
 impl Galley {
+    /// Cursor to the first character.
+    ///
+    /// This is the same as [`Cursor::default`].
+    #[inline]
+    #[allow(clippy::unused_self)]
+    pub fn begin(&self) -> Cursor {
+        Cursor::default()
+    }
+
     /// Cursor to one-past last character.
     pub fn end(&self) -> Cursor {
         if self.rows.is_empty() {