restrict highlight in current page area

This should fix #588.

Highlight in multiple pages is too complicated and requires large
code refactoring and will not be considered for now.

frontend/apps/reader/modules/readerhighlight.lua

@@ -231,6 +231,13 @@ function ReaderHighlight:onHoldPan(arg, ges)
         DEBUG("no previous hold position")
         return true
     end
+    local page_area = self.view:getScreenPageArea(self.hold_pos.page)
+    DEBUG("current page area", page_area)
+    if ges.pos:notIntersectWith(page_area) then
+        DEBUG("not inside page area")
+        return true
+    end
+
     self.holdpan_pos = self.view:screenToPageTransform(ges.pos)
     DEBUG("holdpan position in page", self.holdpan_pos)
     self.selected_text = self.ui.document:getTextFromPositions(self.hold_pos, self.holdpan_pos)