EPUB links: show footnotes in popup, larger tap area (#4261)

Adds new options to the Links> submenu, for now only
available and used with CRE documents.
- Allow larger tap area around links
- Ignore external links
- Show footnotes in popup
- Show more links as footnotes

(This last item is mostly for testing and loosening the
footnote detection algorithm, and see how it would behave
with glossary-like links and inter glossary terms links.)

Fix distance computation from gesture position to link by
using segments.
Code for detecting if a link is a footnote is in cre.cpp, and
tweakable a bit with flags in ReaderLink:showAsFoonotePopup().

Footnotes HTML content is displayed by a new FootnoteWidget,
which uses MuPDF for its rendering.
From it, swipe south or tap outside to close, swipe to the left
to follow the original link and jump to the footnote location
in the book.

Also fix tap on highlights after the recent change to use segments
for displaying: use segments also when checking taps.

frontend/apps/reader/modules/readerlink.lua

@@ -68,23 +68,43 @@ function ReaderLink:initGesListener()
 end
 
 local function isTapToFollowLinksOn()
-    return not G_reader_settings:isFalse("tap_to_follow_links")
+    return G_reader_settings:nilOrTrue("tap_to_follow_links")
+end
+
+local function isLargerTapAreaToFollowLinksEnabled()
+    return G_reader_settings:isTrue("larger_tap_area_to_follow_links")
+end
+
+local function isTapIgnoreExternalLinksEnabled()
+    return G_reader_settings:isTrue("tap_ignore_external_links")
+end
+
+local function isTapLinkFootnotePopupEnabled()
+    return G_reader_settings:isTrue("tap_link_footnote_popup")
+end
+
+local function isPreferFootnoteEnabled()
+    return G_reader_settings:isTrue("link_prefer_footnote")
 end
 
 local function isSwipeToGoBackEnabled()
-    return G_reader_settings:readSetting("swipe_to_go_back") == true
+    return G_reader_settings:isTrue("swipe_to_go_back")
 end
 
 local function isSwipeToFollowFirstLinkEnabled()
-    return G_reader_settings:readSetting("swipe_to_follow_first_link") == true
+    return G_reader_settings:isTrue("swipe_to_follow_first_link")
 end
 
 local function isSwipeToFollowNearestLinkEnabled()
-    return G_reader_settings:readSetting("swipe_to_follow_nearest_link") == true
+    return G_reader_settings:isTrue("swipe_to_follow_nearest_link")
+end
+
+local function isSwipeLinkFootnotePopupEnabled()
+    return G_reader_settings:isTrue("swipe_link_footnote_popup")
 end
 
 local function isSwipeToJumpToLatestBookmarkEnabled()
-    return G_reader_settings:readSetting("swipe_to_jump_to_latest_bookmark") == true
+    return G_reader_settings:isTrue("swipe_to_jump_to_latest_bookmark")
 end
 
 function ReaderLink:addToMainMenu(menu_items)
@@ -150,6 +170,73 @@ If any of the other Swipe to follow link options is enabled, this will work only
             },
         }
     }
+    -- Insert other items that are (for now) only supported with CreDocuments
+    -- (They could be supported nearly as-is, but given that there is a lot
+    -- less visual feedback on PDF document of what is a link, or that we just
+    -- followed a link, than on EPUB, it's safer to not use them on PDF documents
+    -- even if the user enabled these features for EPUB documents).
+    if not self.ui.document.info.has_pages then
+        local footnote_popup_help_text = _([[
+Show internal link target content in a footnote popup when it looks like it might be a footnote, instead of following the link.
+
+Note that depending on the book quality, footnote detection may not always work correctly.
+The footnote content may be empty, truncated, or include other footnotes.
+
+From the footnote popup, you can jump to the footnote location in the book by swiping to the left.]])
+        -- Tap section
+        menu_items.follow_links.sub_item_table[1].separator = nil
+        table.insert(menu_items.follow_links.sub_item_table, 2, {
+            text = _("Allow larger tap area around links"),
+            checked_func = isLargerTapAreaToFollowLinksEnabled,
+            callback = function()
+                G_reader_settings:saveSetting("larger_tap_area_to_follow_links",
+                    not isLargerTapAreaToFollowLinksEnabled())
+            end,
+            help_text = _([[Extends the tap area around internal links. Useful with a small font where tapping on small footnote links may be tedious.]]),
+        })
+        table.insert(menu_items.follow_links.sub_item_table, 3, {
+            text = _("Ignore external links"),
+            checked_func = isTapIgnoreExternalLinksEnabled,
+            callback = function()
+                G_reader_settings:saveSetting("tap_ignore_external_links",
+                    not isTapIgnoreExternalLinksEnabled())
+            end,
+            help_text = _([[Ignore taps on external links. Useful with Wikipedia EPUBs to make page turning easier.
+You can still follow them from the dictionary window or the selection menu after holding on them.]]),
+        })
+        table.insert(menu_items.follow_links.sub_item_table, 4, {
+            text = _("Show footnotes in popup"),
+            checked_func = isTapLinkFootnotePopupEnabled,
+            callback = function()
+                G_reader_settings:saveSetting("tap_link_footnote_popup",
+                    not isTapLinkFootnotePopupEnabled())
+            end,
+            help_text = footnote_popup_help_text,
+            separator = true,
+        })
+        table.insert(menu_items.follow_links.sub_item_table, 5, {
+            text = _("Show more links as footnotes"),
+            checked_func = isPreferFootnoteEnabled,
+            callback = function()
+                G_reader_settings:saveSetting("link_prefer_footnote",
+                    not isPreferFootnoteEnabled())
+            end,
+            help_text = _([[Loosen footnote detection rules to show more links as footnotes.]]),
+            separator = true,
+        })
+        -- Swipe section
+        menu_items.follow_links.sub_item_table[8].separator = nil
+        table.insert(menu_items.follow_links.sub_item_table, 9, {
+            text = _("Show footnotes in popup"),
+            checked_func = isSwipeLinkFootnotePopupEnabled,
+            callback = function()
+                G_reader_settings:saveSetting("swipe_link_footnote_popup",
+                    not isSwipeLinkFootnotePopupEnabled())
+            end,
+            help_text = footnote_popup_help_text,
+            separator = true,
+        })
+    end
     menu_items.go_to_previous_location = {
         text = _("Go back to previous location"),
         enabled_func = function() return #self.location_stack > 0 end,
@@ -220,25 +307,33 @@ function ReaderLink:getLinkFromGes(ges)
         -- element in the same paragraph). If followed then back, we could get
         -- to a different page. So, we check here how valid it is, and if not,
         -- we just discard it so that addCurrentLocationToStack() is used.
-        if a_xpointer and not self:isXpointerCoherent(a_xpointer) then
-            a_xpointer = nil
+        local from_xpointer = nil
+        if a_xpointer and self:isXpointerCoherent(a_xpointer) then
+            from_xpointer = a_xpointer
         end
 
         if link_xpointer ~= "" then
-            -- This link's target xpointer is more precise than a classic
+            -- This link's source xpointer is more precise than a classic
             -- xpointer to top of a page: we can show a marker at its
             -- y-position in target page
+            -- (keep a_xpointer even if incoherent, might be needed for
+            -- footnote detection (better than nothing if incoherent)
             return {
                 xpointer = link_xpointer,
                 marker_xpointer = link_xpointer,
-                from_xpointer = a_xpointer,
+                from_xpointer = from_xpointer,
+                a_xpointer = a_xpointer,
+                -- tap y-position should be a good approximation of link y
+                -- (needed to keep its highlight a bit more time if it was
+                -- hidden by the footnote popup)
+                link_y = ges.pos.y
             }
         end
     end
 end
 
 --- Highlights a linkbox if available and goes to it.
-function ReaderLink:showLinkBox(link)
+function ReaderLink:showLinkBox(link, allow_footnote_popup)
     if link and link.lbox then -- pdfdocument
         -- screen box that holds the link
         local sbox = self.view:pageToScreenTransform(link.pos.page,
@@ -247,12 +342,14 @@ function ReaderLink:showLinkBox(link)
             UIManager:show(LinkBox:new{
                 box = sbox,
                 timeout = FOLLOW_LINK_TIMEOUT,
-                callback = function() self:onGotoLink(link.link) end
+                callback = function()
+                    self:onGotoLink(link.link, false, allow_footnote_popup)
+                end
             })
             return true
         end
     elseif link and link.xpointer ~= "" then -- credocument
-        return self:onGotoLink(link)
+        return self:onGotoLink(link, false, allow_footnote_popup)
     end
 end
 
@@ -265,9 +362,43 @@ end
 
 function ReaderLink:onTap(_, ges)
     if not isTapToFollowLinksOn() then return end
-    local link = self:getLinkFromGes(ges)
-    if link then
-        return self:showLinkBox(link)
+    if self.ui.document.info.has_pages then
+        -- (footnote popup, larger tap area and ignore external links
+        -- are for now not supported with non-CreDocuments)
+        local link = self:getLinkFromGes(ges)
+        if link then
+            return self:showLinkBox(link)
+        end
+        return
+    end
+    local allow_footnote_popup = isTapLinkFootnotePopupEnabled()
+    -- If tap_ignore_external_links, skip precise tap detection to really
+    -- ignore a tap on an external link, and allow using onGoToPageLink()
+    -- to find the nearest internal link
+    if not isTapIgnoreExternalLinksEnabled() then
+        local link = self:getLinkFromGes(ges)
+        if link then
+            return self:showLinkBox(link, allow_footnote_popup)
+        end
+    end
+    if isLargerTapAreaToFollowLinksEnabled() or isTapIgnoreExternalLinksEnabled() then
+        local max_distance = 0 -- used when only isTapIgnoreExternalLinksEnabled()
+        if isLargerTapAreaToFollowLinksEnabled() then
+            -- If no link found exactly at the tap position,
+            -- try to find any link in page around that tap position.
+            -- onGoToPageLink() will grab only internal links, which
+            -- is nice as url links are usually longer - so this
+            -- give more chance to catch a small link to footnote stuck
+            -- to a longer Wikipedia article name link.
+            --
+            -- 30px on a reference 167 dpi screen makes 0.45cm, which
+            -- seems fine (on a 300dpi device, this will be scaled
+            -- to 54px (which makes 1/20th of screen witdh on a GloHD)
+            -- Trust Screen.dpi (which may not be the real device
+            -- screen DPI if the user has set another one).
+            max_distance = Screen:scaleByDPI(30)
+        end
+        return self:onGoToPageLink(ges, false, allow_footnote_popup, max_distance)
     end
 end
 
@@ -283,7 +414,9 @@ function ReaderLink:addCurrentLocationToStack()
 end
 
 --- Goes to link.
-function ReaderLink:onGotoLink(link, neglect_current_location)
+-- (This is called by other modules (highlight, search) to jump to a xpointer,
+-- they should not provide allow_footnote_popup=true)
+function ReaderLink:onGotoLink(link, neglect_current_location, allow_footnote_popup)
     logger.dbg("onGotoLink:", link)
     if self.ui.document.info.has_pages then
         -- internal pdf links have a "page" attribute, while external ones have an "uri" attribute
@@ -305,6 +438,12 @@ function ReaderLink:onGotoLink(link, neglect_current_location)
         -- which accepts both of the above legitimate xpointer as input.
         if self.ui.document:isXPointerInDocument(link.xpointer) then
             logger.dbg("Internal link:", link)
+            if allow_footnote_popup then
+                if self:showAsFootnotePopup(link, neglect_current_location) then
+                    return true
+                end
+                -- if it fails for any reason, fallback to following link
+            end
             if not neglect_current_location then
                 if link.from_xpointer then
                     -- We have a more precise xpointer than the xpointer to top of
@@ -478,9 +617,11 @@ function ReaderLink:onSwipe(arg, ges)
     elseif ges.direction == "west" then
         local ret = false
         if isSwipeToFollowFirstLinkEnabled() then
+            -- no sense allowing footnote popup if first link
             ret = self:onGoToPageLink(ges, true)
         elseif isSwipeToFollowNearestLinkEnabled() then
-            ret = self:onGoToPageLink(ges)
+            local allow_footnote_popup = isSwipeLinkFootnotePopupEnabled()
+            ret = self:onGoToPageLink(ges, false, allow_footnote_popup)
         end
         -- If no link found, or no follow link option enabled,
         -- jump to latest bookmark (if enabled)
@@ -492,8 +633,11 @@ function ReaderLink:onSwipe(arg, ges)
 end
 
 --- Goes to link nearest to the gesture (or first link in page)
-function ReaderLink:onGoToPageLink(ges, use_page_first_link)
+function ReaderLink:onGoToPageLink(ges, use_page_first_link, allow_footnote_popup, max_distance)
     local selected_link = nil
+    local selected_distance2 = nil
+    -- We use squares of distances all along the calculations, no need
+    -- to math.sqrt() them when comparing
     if self.ui.document.info.has_pages then
         local pos = self.view:screenToPageTransform(ges.pos)
         if not pos then
@@ -539,8 +683,18 @@ function ReaderLink:onGoToPageLink(ges, use_page_first_link)
                 end
             end
         end
+        if shortest_dist then
+            selected_distance2 = shortest_dist
+        end
     else
-        local links = self.ui.document:getPageLinks()
+        -- Getting segments on a page with many internal links is
+        -- a bit expensive. With larger_tap_area_to_follow_links=true,
+        -- this is done for each tap on screen (changing pages, showing
+        -- menu...). We might want to cache these links per page (and
+        -- clear that cache when page layout change).
+        -- As we care only about internal links, we request them only
+        -- (and avoid that expensive segments work on external links)
+        local links = self.ui.document:getPageLinks(true) -- only get internal links
         if not links or #links == 0 then
             return
         end
@@ -563,6 +717,29 @@ function ReaderLink:onGoToPageLink(ges, use_page_first_link)
         --         ["start_y"] = 1201
         --         ["a_xpointer"] = "/body/DocFragment/body/div/p[12]/sup[3]/a[3].0",
         --     },
+        -- and when segments requested (example for a multi-lines link):
+        --     [3] = {
+        --         ["section"] = "#_doc_fragment_0_ Man_of_letters",
+        --         ["a_xpointer"] = "/body/DocFragment/body/div/div[4]/ul/li[3]/ul/li[2]/ul/li[1]/ul/li[3]/a.0",
+        --         ["start_x"] = 101,
+        --         ["start_y"] = 457,
+        --         ["end_x"] = 176,
+        --         ["end_y"] = 482,,
+        --         ["segments"] = {
+        --             [1] = {
+        --                  ["x0"] = 101,
+        --                  ["y0"] = 457,
+        --                  ["x1"] = 590,
+        --                  ["y1"] = 482,
+        --             },
+        --             [2] = {
+        --                  ["x0"] = 101,
+        --                  ["y0"] = 482,
+        --                  ["x1"] = 177,
+        --                  ["y1"] = 507,
+        --             }
+        --         },
+        --     },
         -- Note: with some documents and some links, crengine may give wrong
         -- coordinates, and our code below may miss or give the wrong first
         -- or nearest link...
@@ -579,42 +756,96 @@ function ReaderLink:onGoToPageLink(ges, use_page_first_link)
                         first_start_y = link["start_y"]
                     end
                 else
-                    local start_dist = math.pow(link.start_x - pos_x, 2) + math.pow(link.start_y - pos_y, 2)
-                    local end_dist = math.pow(link.end_x - pos_x, 2) + math.pow(link.end_y - pos_y, 2)
-                    local min_dist = math.min(start_dist, end_dist)
-                    if shortest_dist == nil or min_dist < shortest_dist then
-                        selected_link = link
-                        shortest_dist = min_dist
+                    if link["segments"] then
+                        -- With segments, each is a horizontal segment, with start_x < end_x,
+                        -- and we should compute the distance from gesture position to
+                        -- each segment.
+                        local segments_max_y = -1
+                        local link_is_shortest = false
+                        local segments = link["segments"]
+                        for i=1, #segments do
+                            local segment = segments[i]
+                            local segment_dist
+                            -- Distance here is kept squared (d^2 = diff_x^2 + diff_y^2),
+                            -- and we compute each part individually
+                            -- First, vertical distance (squared)
+                            if pos_y < segment.y0 then -- above the segment height
+                                segment_dist = math.pow(segment.y0 - pos_y, 2)
+                            elseif pos_y > segment.y1 then -- below the segment height
+                                segment_dist = math.pow(pos_y - segment.y1, 2)
+                            else -- gesture pos is on the segment height, no vertical distance
+                                segment_dist = 0
+                            end
+                            -- Next, horizontal distance (squared)
+                            if pos_x < segment.x0 then -- on the left of segment: calc dist to x0
+                                segment_dist = segment_dist + math.pow(segment.x0 - pos_x, 2)
+                            elseif pos_x > segment.x1 then -- on the right of segment : calc dist to x1
+                                segment_dist = segment_dist + math.pow(pos_x - segment.x1, 2)
+                            -- else -- gesture pos is in the segment width, no horizontal distance
+                            end
+                            if shortest_dist == nil or segment_dist < shortest_dist then
+                                selected_link = link
+                                shortest_dist = segment_dist
+                                link_is_shortest = true
+                            end
+                            if segment.y1 > segments_max_y then
+                                segments_max_y = segment.y1
+                            end
+                        end
+                        if link_is_shortest then
+                            -- update the selected_link we just set with its lower segment y
+                            selected_link.link_y = segments_max_y
+                        end
+                    else
+                        -- Before "segments" were available, we did this:
+                        -- We'd only get a horizontal segment if the link is on a single line.
+                        -- When it is multi-lines, we can't do much calculation...
+                        -- We used to just check distance from start_x and end_x, and
+                        -- we could miss a tap in the middle of a long link.
+                        -- (also start_y = end_y = the top of the rect for a link on a single line)
+                        local start_dist = math.pow(link.start_x - pos_x, 2) + math.pow(link.start_y - pos_y, 2)
+                        local end_dist = math.pow(link.end_x - pos_x, 2) + math.pow(link.end_y - pos_y, 2)
+                        local min_dist = math.min(start_dist, end_dist)
+                        if shortest_dist == nil or min_dist < shortest_dist then
+                            selected_link = link
+                            selected_link.link_y = link.end_y
+                            shortest_dist = min_dist
+                        end
                     end
                 end
             end
         end
-        -- cre.cpp getPageLinks() does highlight found links :
-        --   sel.add( new ldomXRange(*links[i]) ); // highlight
-        -- and we'll find them highlighted when back from link.
-        -- So let's clear them now.
-        self.ui.document:clearSelection()
-        -- (Comment out previous line to visually see which links on the
-        -- page are not coherent: those not highlighted)
+        if shortest_dist then
+            selected_distance2 = shortest_dist
+        end
 
         if selected_link then
-            logger.dbg("original selected_link", selected_link)
+            logger.dbg("nearest selected_link", selected_link)
+            -- Check a_xpointer is coherent, use it as from_xpointer only if it is
+            local from_xpointer = nil
+            if selected_link.a_xpointer and self:isXpointerCoherent(selected_link.a_xpointer) then
+                from_xpointer = selected_link.a_xpointer
+            end
             -- Make it a link as expected by onGotoLink
             selected_link = {
-                xpointer = selected_link["section"],
-                marker_xpointer = selected_link["section"],
-                from_xpointer = selected_link["a_xpointer"],
+                xpointer = selected_link.section,
+                marker_xpointer = selected_link.section,
+                from_xpointer = from_xpointer,
+                -- (keep a_xpointer even if incoherent, might be needed for
+                -- footnote detection (better than nothing if incoherent)
+                a_xpointer = selected_link.a_xpointer,
+                -- keep the link y position, so we can keep its highlight shown
+                -- a bit more time if it was hidden by the footnote popup
+                link_y = selected_link.link_y,
             }
-            logger.dbg("selected_link", selected_link)
-            -- Check from_xpointer is coherent, and unset it if not
-            if selected_link.from_xpointer and not self:isXpointerCoherent(selected_link.from_xpointer) then
-                selected_link.from_xpointer = nil
-            end
-
         end
     end
     if selected_link then
-        return self:onGotoLink(selected_link)
+        if max_distance and selected_distance2 and selected_distance2 > math.pow(max_distance, 2) then
+            logger.dbg("nearest link is further than max distance, ignoring it")
+        else
+            return self:onGotoLink(selected_link, false, allow_footnote_popup)
+        end
     end
 end
 
@@ -647,4 +878,188 @@ function ReaderLink:onGoToLatestBookmark(ges)
     end
 end
 
+function ReaderLink:showAsFootnotePopup(link, neglect_current_location)
+    if self.ui.document.info.has_pages then
+        return false -- not supported
+    end
+
+    local source_xpointer = link.from_xpointer or link.a_xpointer
+    local target_xpointer = link.xpointer
+    if not source_xpointer or not target_xpointer then
+        return false
+    end
+    local trust_source_xpointer = link.from_xpointer ~= nil
+
+    -- For reference, Kobo information and conditions for showing a link as popup:
+    --   https://github.com/kobolabs/epub-spec#footnotesendnotes-are-fully-supported-across-kobo-platforms
+    -- Calibre has its own heuristics to decide if a link is to a footnote or not,
+    -- and what to gather around the footnote target as the footnote content to display:
+    -- Nearly same logic, implemented in python and in coffeescript:
+    --   https://github.com/kovidgoyal/calibre/blob/master/src/pyj/read_book/footnotes.pyj
+    --   https://github.com/kovidgoyal/calibre/blob/master/src/calibre/ebooks/oeb/display/extract.coffee
+
+    -- We do many tests, including most of those done by Kobo and Calibre, to
+    -- detect if a link is to a footnote.
+    -- The detection is done in cre.cpp, because it makes use of DOM navigation and
+    -- inspection that can't be done from Lua (unless we add many proxy functions)
+
+    -- Detection flags, to allow tweaking a bit cre.cpp code if needed
+    local flags = 0
+
+    -- If no detection decided, fallback to false (not a footnote, so, follow link)
+    if isPreferFootnoteEnabled() then
+        flags = flags + 0x0001 -- if set, fallback to true
+    end
+
+    if trust_source_xpointer then
+        -- trust source_xpointer: allow checking attribute and styles
+        -- if not trusted, checks marked (*) don't apply
+        flags = flags + 0x0002
+    end
+
+    -- Trust role= and epub:type= attribute values if defined, for source(*) and target
+    -- (If needed, we could add a check for a private CSS property "-cr-hint: footnote"
+    -- or "-cr-hint: noteref", so one can define it to specific classes with Styles
+    -- tweaks.)
+    flags = flags + 0x0004
+    -- flags = flags + 0x0008 -- Unused yet
+
+    -- TARGET STATUS AND SOURCE RELATION
+    -- Target must have an anchor #id (ie: must not be a simple link to a html file)
+    flags = flags + 0x0010
+    -- Target must come after source in the book
+    -- (Glossary definitions may point to others before, so avoid this check
+    -- if user wants more footnotes)
+    if not isPreferFootnoteEnabled() then
+        flags = flags + 0x0020
+    end
+    -- Target must not be a target of a TOC entry
+    flags = flags + 0x0040
+    -- flags = flags + 0x0080 -- Unused yet
+
+    -- SOURCE NODE CONTEXT
+    -- (*) Source link must not be empty content, and must not be the only content of
+    -- its parent block tag (this could mean it's a chapter title in an inline ToC)
+    flags = flags + 0x0100
+    -- (*) Source node vertical alignment:
+    -- check that all non-empty-nor-space-only children have their computed
+    -- vertical-align: any of: sub super top bottom (which will be the case
+    -- whether a parent or the childre themselves are in a <sub> or <sup>)
+    -- (Also checks if parent or children are <sub> or <sup>, which may
+    -- have been tweaked with CSS to not have one of these vertical-align.)
+    flags = flags + 0x0200
+    -- (*) Source node text (punctuation and parens stripped) is a number
+    -- (3 digits max, to avoid catching years ... but only years>1000)
+    flags = flags + 0x0400
+    -- (*) Source node text (punctuation and parens stripped) is 1 or 2 letters,
+    -- with 0 to 2 numbers (a, z, ab, 1a, B2)
+    flags = flags + 0x0800
+
+    -- TARGET NODE CONTEXT
+    -- Target must not contain, or be contained, in H1..H6
+    flags = flags + 0x1000
+    -- flags = flags + 0x2000 -- Unused yet
+    -- Try to extend footnote, to gather more text after target
+    flags = flags + 0x4000
+    -- Extended target readable text (not accounting HTML tags) must not be
+    -- larger than max_text_size
+    flags = flags + 0x8000
+    local max_text_size = 10000 -- nb of chars
+
+    logger.dbg("Checking if link is to a footnote:", flags, source_xpointer, target_xpointer)
+    local is_footnote, reason, extStopReason, extStartXP, extEndXP =
+            self.ui.document:isLinkToFootnote(source_xpointer, target_xpointer, flags, max_text_size)
+    if not is_footnote then
+        logger.info("not a footnote:", reason)
+        return false
+    end
+    logger.info("is a footnote:", reason)
+    if extStartXP then
+        logger.info("  extended until:", extStopReason)
+        logger.info(extStartXP)
+        logger.info(extEndXP)
+    else
+        logger.info("  not extended because:", extStopReason)
+    end
+    -- OK, done with the dirty footnote detection work, we can now
+    -- get back to the fancy UI stuff
+
+    -- We don't request CSS files, to have a more consistent footnote style.
+    -- (we still get and give to MuPDF styles set with style="" )
+    -- (We also don't because MuPDF is quite sensitive to bad css, and may
+    -- then just ignore the whole stylesheet, including our own declarations
+    -- we add at start)
+    --
+    -- flags = 0x0000 to get the simplest/purest HTML without CSS
+    local html
+    if extStartXP and extEndXP then
+        html = self.ui.document:getHTMLFromXPointers(extStartXP, extEndXP, 0x0000)
+    else
+        html = self.ui.document:getHTMLFromXPointer(target_xpointer, 0x0000, true)
+        -- from_final_parent = true to get a possibly more complete footnote
+    end
+    if not html then
+        logger.info("failed getting HTML for xpointer:", target_xpointer)
+        return false
+    end
+
+    -- if false then -- for debug, to display html
+    --     UIManager:show( require("ui/widget/textviewer"):new{text = html})
+    --     return true
+    -- end
+
+    -- As we stay on the current page, we can highlight the selected link
+    -- (which might not be seen when covered by FootnoteWidget)
+    local close_callback = nil
+    if link.from_xpointer then -- coherent xpointer
+        self.ui.document:highlightXPointer(link.from_xpointer)
+        UIManager:setDirty(self.dialog, "ui")
+        close_callback = function(footnote_height)
+            -- remove this highlight (actually all) on close
+            local clear_highlight = function()
+                self.ui.document:highlightXPointer()
+                UIManager:setDirty(self.dialog, "ui")
+            end
+            if footnote_height then
+                -- If the link was hidden by the footnote popup,
+                -- delay a bit its clearing, so the user can see
+                -- it and know where to start reading again
+                local footnote_top_y = Screen:getHeight() - footnote_height
+                if link.link_y > footnote_top_y then
+                    UIManager:scheduleIn(0.5, clear_highlight)
+                else
+                    clear_highlight()
+                end
+            else
+                clear_highlight()
+            end
+        end
+    end
+
+    -- We give FootnoteWidget the document margins and font size, so
+    -- it can base its own values on them (note that this can look
+    -- misaligned when floating punctuation is enabled, as margins then
+    -- don't have a fixed size)
+    local FootnoteWidget = require("ui/widget/footnotewidget")
+    local popup
+    popup = FootnoteWidget:new{
+        html = html,
+        doc_font_size = Screen:scaleBySize(self.ui.font.font_size),
+        doc_margins = self.ui.document._document:getPageMargins(),
+        close_callback = close_callback,
+        follow_callback = function() -- follow the link on swipe west
+            UIManager:close(popup)
+            self:onGotoLink(link, neglect_current_location)
+        end,
+        on_tap_close_callback = function(arg, ges)
+            -- on tap outside, see if we are tapping on another footnote,
+            -- and display it if we do (avoid the need for 2 taps)
+            self:onTap(arg, ges)
+        end,
+        dialog = self.dialog,
+    }
+    UIManager:show(popup)
+    return true
+end
+
 return ReaderLink