E-book viewer: When using a right click/shift-click to adjust the selection, move the section boundary that is closer to the click point. Fixes #1929164 [E-book viewer: Right- and Shift-clicking inside the selection always change its lower boundary](https://bugs.launchpad.net/calibre/+bug/1929164)

src/pyj/read_book/iframe.pyj

@@ -67,7 +67,8 @@
 )
 from read_book.viewport import scroll_viewport
 from select import (
-    first_visible_word, move_end_of_selection, selection_extents, word_at_point
+    first_visible_word, is_start_closer_to_point, move_end_of_selection,
+    selection_extents, word_at_point
 )
 from utils import debounce, is_ios
 
@@ -809,7 +810,7 @@ def annotations_msg_received(self, data):
             except:
                 (console.error or console.log)('Failed to extend selection to paragraph')
         elif dtype is 'extend-to-point':
-            move_end_of_selection(data.pos, None)
+            move_end_of_selection(data.pos, is_start_closer_to_point(data.pos))
         elif dtype is 'drag-scroll':
             self.scroll_to_extend_annotation(data.backwards)
         elif dtype is 'edit-highlight':

src/pyj/select.pyj

@@ -190,6 +190,23 @@ def selection_extents(in_flow_mode):
     return range_extents(sel.getRangeAt(0), in_flow_mode)
 
 
+def is_start_closer_to_point(pos):
+    sel = window.getSelection()
+    if not sel.rangeCount:
+        return
+    p = caret_position_from_point(pos.x, pos.y)
+    if p:
+        r = sel.getRangeAt(0)
+        sr = document.createRange()
+        sr.setStart(r.startContainer, r.startOffset)
+        sr.setEnd(p.offsetNode, p.offset)
+        er = document.createRange()
+        er.setStart(p.offsetNode, p.offset)
+        er.setEnd(r.endContainer, r.endOffset)
+        return sr.toString().length < er.toString().length
+    return False
+
+
 def move_end_of_selection(pos, start):
     sel = window.getSelection()
     if not sel.rangeCount: