LibGUI: Convert mouse events from a visual to a physical position

When clicking a position within a TextEditor, we should interpret that
position as a visual location. That location should be converted to a
"physical" location before using it to set the physical cursor position.

For example, consider a document with 2 emoji, each consisting of 3 code
points. Visually, these will occupy 2 columns. When a mouse click occurs
between these columns, we need to convert the visual column number 1 to
the physical column number 3 when storing the new cursor location.

Userland/Libraries/LibGUI/TextEditor.cpp

@@ -33,6 +33,7 @@
 #include <LibGfx/Palette.h>
 #include <LibGfx/StandardCursor.h>
 #include <LibSyntax/Highlighter.h>
+#include <LibUnicode/Segmentation.h>
 #include <fcntl.h>
 #include <stdio.h>
 #include <unistd.h>
@@ -276,7 +277,7 @@ void TextEditor::mousedown_event(MouseEvent& event)
 
     m_in_drag_select = true;
 
-    set_cursor(text_position_at(event.position()));
+    set_cursor_to_text_position(event.position());
 
     if (!(event.modifiers() & Mod_Shift)) {
         if (!has_selection())
@@ -306,7 +307,7 @@ void TextEditor::mousemove_event(MouseEvent& event)
     m_last_mousemove_position = event.position();
     if (m_in_drag_select) {
         auto constrained = event.position().constrained(widget_inner_rect());
-        set_cursor(text_position_at(constrained));
+        set_cursor_to_text_position(constrained);
         m_selection.set_end(m_cursor);
         did_update_selection();
         update();
@@ -336,7 +337,7 @@ void TextEditor::automatic_scrolling_timer_did_fire()
         set_automatic_scrolling_timer_active(false);
         return;
     }
-    set_cursor(text_position_at(m_last_mousemove_position));
+    set_cursor_to_text_position(m_last_mousemove_position);
     m_selection.set_end(m_cursor);
     did_update_selection();
     update();
@@ -1395,6 +1396,25 @@ void TextEditor::set_cursor(TextPosition const& a_position)
         update();
 }
 
+void TextEditor::set_cursor_to_text_position(Gfx::IntPoint position)
+{
+    auto visual_position = text_position_at(position);
+    size_t physical_column = 0;
+
+    auto const& line = document().line(visual_position.line());
+    size_t boundary_index = 0;
+
+    Unicode::for_each_grapheme_segmentation_boundary(line.view(), [&](auto boundary) {
+        physical_column = boundary;
+
+        if (boundary_index++ >= visual_position.column())
+            return IterationDecision::Break;
+        return IterationDecision::Continue;
+    });
+
+    set_cursor({ visual_position.line(), physical_column });
+}
+
 void TextEditor::focusin_event(FocusEvent& event)
 {
     if (event.source() == FocusSource::Keyboard)

Userland/Libraries/LibGUI/TextEditor.h

@@ -190,6 +190,7 @@ public:
     void set_cursor_and_focus_line(size_t line, size_t column);
     void set_cursor(size_t line, size_t column);
     virtual void set_cursor(TextPosition const&);
+    void set_cursor_to_text_position(Gfx::IntPoint);
 
     Syntax::Highlighter* syntax_highlighter();
     Syntax::Highlighter const* syntax_highlighter() const;