matplotlib · tacaswell · Oct 20, 2022 · Oct 1, 2022
diff --git a/lib/matplotlib/tests/test_text.py b/lib/matplotlib/tests/test_text.py
@@ -339,6 +339,32 @@ def test_set_position():
         assert a + shift_val == b
 
 
+def test_char_index_at():
+    fig = plt.figure()
+    text = fig.text(0.1, 0.9, "")
+
+    text.set_text("i")
+    bbox = text.get_window_extent()
+    size_i = bbox.x1 - bbox.x0
+
+    text.set_text("m")
+    bbox = text.get_window_extent()
+    size_m = bbox.x1 - bbox.x0
+
+    text.set_text("iiiimmmm")
+    bbox = text.get_window_extent()
+    origin = bbox.x0
+
+    assert text._char_index_at(origin - size_i) == 0  # left of first char
+    assert text._char_index_at(origin) == 0
+    assert text._char_index_at(origin + 0.499*size_i) == 0
+    assert text._char_index_at(origin + 0.501*size_i) == 1
+    assert text._char_index_at(origin + size_i*3) == 3
+    assert text._char_index_at(origin + size_i*4 + size_m*3) == 7
+    assert text._char_index_at(origin + size_i*4 + size_m*4) == 8
+    assert text._char_index_at(origin + size_i*4 + size_m*10) == 8
+
+
 @pytest.mark.parametrize('text', ['', 'O'], ids=['empty', 'non-empty'])
 def test_non_default_dpi(text):
     fig, ax = plt.subplots()

diff --git a/lib/matplotlib/text.py b/lib/matplotlib/text.py
@@ -124,6 +124,7 @@ class Text(Artist):
     """Handle storing and drawing of text in window or data coordinates."""
 
     zorder = 3
+    _charsize_cache = dict()
 
     def __repr__(self):
         return "Text(%s, %s, %s)" % (self._x, self._y, repr(self._text))
@@ -279,6 +280,38 @@ def _get_multialignment(self):
         else:
             return self._horizontalalignment
 
+    def _char_index_at(self, x):
+        """
+        Calculate the index closest to the coordinate x in display space.
+
+        The position of text[index] is assumed to be the sum of the widths
+        of all preceding characters text[:index].
+
+        This works only on single line texts.
+        """
+        if not self._text:
+            return 0
+
+        text = self._text
+
+        fontproperties = str(self._fontproperties)
+        if fontproperties not in Text._charsize_cache:
+            Text._charsize_cache[fontproperties] = dict()
+
+        charsize_cache = Text._charsize_cache[fontproperties]
+        for char in set(text):
+            if char not in charsize_cache:
+                self.set_text(char)
+                bb = self.get_window_extent()
+                charsize_cache[char] = bb.x1 - bb.x0
+
+        self.set_text(text)
+        bb = self.get_window_extent()
+
+        size_accum = np.cumsum([0] + [charsize_cache[x] for x in text])
+        std_x = x - bb.x0
+        return (np.abs(size_accum - std_x)).argmin()
+
     def get_rotation(self):
         """Return the text angle in degrees between 0 and 360."""
         if self.get_transform_rotates_text():

diff --git a/lib/matplotlib/widgets.py b/lib/matplotlib/widgets.py
@@ -1315,17 +1315,6 @@ def stop_typing(self):
             # call it once we've already done our cleanup.
             self._observers.process('submit', self.text)
 
-    def position_cursor(self, x):
-        # now, we have to figure out where the cursor goes.
-        # approximate it based on assuming all characters the same length
-        if len(self.text) == 0:
-            self.cursor_index = 0
-        else:
-            bb = self.text_disp.get_window_extent()
-            ratio = np.clip((x - bb.x0) / bb.width, 0, 1)
-            self.cursor_index = int(len(self.text) * ratio)
-        self._rendercursor()
-
     def _click(self, event):
         if self.ignore(event):
             return
@@ -1338,7 +1327,8 @@ def _click(self, event):
             event.canvas.grab_mouse(self.ax)
         if not self.capturekeystrokes:
             self.begin_typing(event.x)
-        self.position_cursor(event.x)
+        self.cursor_index = self.text_disp._char_in
4A42
dex_at(event.x)
+        self._rendercursor()
 
     def _resize(self, event):
         self.stop_typing()