Footnote popups: allow setting an absolute font size

In the "Set footnote popup font size", allow toggling
between setting a relative (to the document) font size
and setting an absolute font size (that won't change
with the document font size).

frontend/apps/reader/modules/readerlink.lua

@@ -271,8 +271,35 @@ From the footnote popup, you can jump to the footnote location in the book by sw
             end,
             keep_menu_open = true,
             callback = function()
+                local spin_widget
+                local get_font_size_widget
+                get_font_size_widget = function(show_absolute_font_size_widget)
                     local SpinWidget = require("ui/widget/spinwidget")
-                UIManager:show(SpinWidget:new{
+                    if show_absolute_font_size_widget then
+                        spin_widget = SpinWidget:new{
+                            width = math.floor(Screen:getWidth() * 0.75),
+                            value = G_reader_settings:readSetting("footnote_popup_absolute_font_size")
+                                            or Screen:scaleBySize(self.ui.font.font_size),
+                            value_min = 12,
+                            value_max = 255,
+                            precision = "%d",
+                            ok_text = _("Set font size"),
+                            title_text =  _("Set footnote popup font size"),
+                            info_text = _([[
+The footnote popup font can adjust to the font size you've set for the document, but you can specify here a fixed absolute font size to be used instead.]]),
+                            callback = function(spin)
+                                G_reader_settings:delSetting("footnote_popup_relative_font_size")
+                                G_reader_settings:saveSetting("footnote_popup_absolute_font_size", spin.value)
+                            end,
+                            extra_text = _("Set a relative font size instead"),
+                            extra_callback = function()
+                                UIManager:close(spin_widget)
+                                spin_widget = get_font_size_widget(false)
+                                UIManager:show(spin_widget)
+                            end,
+                        }
+                    else
+                        spin_widget = SpinWidget:new{
                             width = math.floor(Screen:getWidth() * 0.75),
                             value = G_reader_settings:readSetting("footnote_popup_relative_font_size") or -2,
                             value_min = -10,
@@ -286,9 +313,22 @@ You can specify here how much smaller or larger it should be relative to the doc
 A negative value will make it smaller, while a positive one will make it larger.
 The recommended value is -2.]]),
                             callback = function(spin)
+                                G_reader_settings:delSetting("footnote_popup_absolute_font_size")
                                 G_reader_settings:saveSetting("footnote_popup_relative_font_size", spin.value)
                             end,
-                })
+                            extra_text = _("Set an absolute font size instead"),
+                            extra_callback = function()
+                                UIManager:close(spin_widget)
+                                spin_widget = get_font_size_widget(true)
+                                UIManager:show(spin_widget)
+                            end,
+                        }
+                    end
+                    return spin_widget
+                end
+                local show_absolute_font_size_widget = G_reader_settings:readSetting("footnote_popup_absolute_font_size") ~= nil
+                spin_widget = get_font_size_widget(show_absolute_font_size_widget)
+                UIManager:show(spin_widget)
             end,
             help_text = _([[
 The footnote popup font adjusts to the font size you've set for the document.

frontend/ui/widget/footnotewidget.lua

@@ -225,7 +225,12 @@ function FootnoteWidget:init()
     -- We may use a font size a bit smaller than the document one (because
     -- footnotes are usually smaller, and because NotoSans is a bit on the
     -- larger size when compared to other fonts at the same size)
-    local font_size = self.doc_font_size + (G_reader_settings:readSetting("footnote_popup_relative_font_size") or -2)
+    local font_size = G_reader_settings:readSetting("footnote_popup_absolute_font_size")
+    if font_size then
+        font_size = Screen:scaleBySize(font_size)
+    else
+        font_size = self.doc_font_size + (G_reader_settings:readSetting("footnote_popup_relative_font_size") or -2)
+    end
 
     -- We want to display the footnote text with the same margins as
     -- the document, but keep the scrollbar in the right margin, so