Correctly locate cursor in hypertext widgets

The gertty-specific hypertext widget did not handle cursor positioning,
so selectable text did not behave like buttons.  This is an inconsistent
UI and causes problems for screen readers.

Change-Id: Iede3f663775fc169a757751acc1fe752dc00f544

gertty/app.py

@@ -34,6 +34,8 @@ from six.moves import queue
 from six.moves.urllib import parse as urlparse
 import sqlalchemy.exc
 import urwid
+# To aid debugging cursor positions
+# urwid.escape.HIDE_CURSOR=''
 
 from gertty import db
 from gertty import config

gertty/mywid.py

@@ -480,6 +480,22 @@ class HyperText(urwid.Text):
         data['pos'] += len(markup)
         return markup
 
+    def get_cursor_coords(self, size):
+        if self.focused_index is None:
+            cursor_pos = 0
+            return None
+        else:
+            item, start, end = self.selectable_items[self.focused_index]
+            cursor_pos = start
+
+        (maxcol,) = size
+        trans = self.get_line_translation(maxcol)
+        x, y = urwid.text_layout.calc_coords(self.text, trans, cursor_pos)
+        if maxcol <= x:
+            return None
+
+        return x, y
+
     def set_text(self, markup):
         self._markup = markup
         self.selectable_items = []
@@ -496,7 +512,11 @@ class HyperText(urwid.Text):
     def render(self, size, focus=False):
         if (not focus) and (self.focused_index is not None):
             self.focusItem(None)
-        return super(HyperText, self).render(size, focus)
+        ret = super(HyperText, self).render(size, focus)
+        if focus:
+            ret = urwid.canvas.CompositeCanvas(ret)
+            ret.cursor = self.get_cursor_coords(size)
+        return ret
 
 class Link(urwid.Widget):
     signals = ['selected']