Don't calculate widths per-uft8-char when not needed (#1409)

data/core/docview.lua

@@ -175,14 +175,23 @@ function DocView:get_col_x_offset(line, col)
   for _, type, text in self.doc.highlighter:each_token(line) do
     local font = style.syntax_fonts[type] or default_font
     if font ~= default_font then font:set_tab_size(indent_size) end
+    local length = #text
+    if column + length <= col then
+      xoffset = xoffset + font:get_width(text)
+      column = column + length
+      if column >= col then
+        return xoffset
+      end
+    else
       for char in common.utf8_chars(text) do
-      if column == col then
+        if column >= col then
           return xoffset
         end
         xoffset = xoffset + font:get_width(char)
         column = column + #char
       end
     end
+  end
 
   return xoffset
 end
@@ -198,6 +207,13 @@ function DocView:get_x_offset_col(line, x)
   for _, type, text in self.doc.highlighter:each_token(line) do
     local font = style.syntax_fonts[type] or default_font
     if font ~= default_font then font:set_tab_size(indent_size) end
+    local width = font:get_width(text)
+    -- Don't take the shortcut if the width matches x,
+    -- because we need last_i which should be calculated using utf-8.
+    if xoffset + width < x then
+      xoffset = xoffset + width
+      i = i + #text
+    else
       for char in common.utf8_chars(text) do
         local w = font:get_width(char)
         if xoffset >= x then
@@ -208,6 +224,7 @@ function DocView:get_x_offset_col(line, x)
         i = i + #char
       end
     end
+  end
 
   return #line_text
 end