E-book viewer: Allow using shift+click to extend selections. Fixes #1918251 [E-book viewer selection broken](https://bugs.launchpad.net/calibre/+bug/1918251)

manual/viewer.rst

@@ -111,6 +111,8 @@ highlight. You can add notes and change the color of the highlight. On a touch
 screen, long tap a word to select it and show the popup bar. Once in highlight
 mode you can change what text is selected, using touch screen friendly selection
 handles. Drag the handles to the top or bottom margins to scroll while selecting.
+You can also hold the :kbd:`Shift` key and click to extend the selection,
+particularly useful for multi-page selections.
 
 You can use the :guilabel:`Highlights` button in the viewer
 controls to show a separate panel with a list of all highlights in the book,

src/pyj/read_book/iframe.pyj

@@ -810,6 +810,8 @@ class IframeBoss:
                 sel.setBaseAndExtent(sel.focusNode, sel.focusOffset, end_node, end_offset)
             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)
         elif dtype is 'drag-scroll':
             self.scroll_to_extend_annotation(data.backwards)
         elif dtype is 'edit-highlight':

src/pyj/read_book/selection_bar.pyj

@@ -480,6 +480,11 @@ class SelectionBar:
             self.hide_editor(True)
         if self.state is WAITING:
             now = window.performance.now()
+            if ev.shiftKey:
+                pos = {'x': ev.clientX, 'y': ev.clientY}
+                map_to_iframe_coords(pos, get_margins())
+                self.send_message('extend-to-point', pos=pos)
+                return
             if self.last_double_click_at and now - self.last_double_click_at < 500:
                 self.send_message('extend-to-paragraph')
                 return

src/pyj/select.pyj

@@ -197,6 +197,16 @@ def move_end_of_selection(pos, start):
     p = caret_position_from_point(pos.x, pos.y)
     if p:
         r = sel.getRangeAt(0)
+        if start is None:
+            q = document.createRange()
+            q.setStart(p.offsetNode, p.offset)
+            q.setEnd(p.offsetNode, p.offset)
+            if r.compareBoundaryPoints(window.Range.START_TO_START, q) >= 0:
+                start = True
+            elif r.compareBoundaryPoints(window.Range.END_TO_END, q) <= 0:
+                start = False
+            else:
+                return
         if start:
             if r.startContainer is not p.offsetNode or r.startOffset is not p.offset:
                 r.setStart(p.offsetNode, p.offset)