Reader: Allow scrolling small offsets in continuous mode.

Using the drag gestures is cumbersome and finicky, especially when
you want to do it all the time. Thus, we add a new option here to
modify the default tapping behavior.

frontend/apps/reader/modules/readerpaging.lua

@@ -145,7 +145,7 @@ function ReaderPaging:setupTouchZones()
             screen_zone = forward_zone,
             handler = function()
                 if G_reader_settings:nilOrFalse("page_turns_disable_tap") then
-                    return self:onGotoViewRel(1)
+                    return self:onPageTurnRel(1)
                 end
             end,
         },
@@ -155,7 +155,7 @@ function ReaderPaging:setupTouchZones()
             screen_zone = backward_zone,
             handler = function()
                 if G_reader_settings:nilOrFalse("page_turns_disable_tap") then
-                    return self:onGotoViewRel(-1)
+                    return self:onPageTurnRel(-1)
                 end
             end,
         },
@@ -191,6 +191,14 @@ function ReaderPaging:onReadSettings(config)
     self:_gotoPage(config:readSetting("last_page") or 1)
     self.flipping_zoom_mode = config:readSetting("flipping_zoom_mode") or "page"
     self.flipping_scroll_mode = config:isTrue("flipping_scroll_mode")
+    self.page_scroll_amount = config:readSetting("kopt_page_scroll_amount")
+        or G_reader_settings:readSetting("kopt_page_scroll_amount")
+        or 0
+end
+
+function ReaderPaging:onPageScrollAmountUpdate(page_scroll_amount)
+    self.page_scroll_amount = page_scroll_amount
+    return true
 end
 
 function ReaderPaging:onSaveSettings()
@@ -579,6 +587,14 @@ function ReaderPaging:onGotoViewRel(diff)
     return true
 end
 
+function ReaderPaging:onPageTurnRel(diff)
+    -- handle smooth scroll by default.
+    if self.view.page_scroll and self.page_scroll_amount ~= 0 then
+        return self:onGotoPosRel(self.page_scroll_amount * diff)
+    end
+    return self:onGotoViewRel(diff)
+end
+
 function ReaderPaging:onGotoPosRel(diff)
     if self.view.page_scroll then
         self:onPanningRel(100*diff)

frontend/dispatcher.lua

@@ -239,6 +239,7 @@ local settingsList = {
     kopt_zoom_direction = {category="string", paging=true},
     kopt_page_scroll = {category="string", paging=true},
     kopt_page_gap_height = {category="string", paging=true},
+    kopt_page_scroll_amount = {category="string", paging=true},
     kopt_full_screen = {category="string", paging=true},
     kopt_line_spacing = {category="configurable", paging=true},
     kopt_justification = {category="configurable", paging=true},
@@ -451,6 +452,7 @@ local dispatcher_menu_order = {
     "kopt_zoom_direction",
     "kopt_page_scroll",
     "kopt_page_gap_height",
+    "kopt_page_scroll_amount",
     "kopt_full_screen",
     "kopt_line_spacing",
     "kopt_justification",

frontend/ui/data/koptoptions.lua

@@ -352,6 +352,20 @@ left to right or reverse, top to bottom or reverse.]]),
                 name_text_hold_callback = optionsutil.showValues,
                 help_text = _([[In continuous view mode, sets the thickness of the separator between document pages.]]),
             },
+            {
+                name = "page_scroll_amount",
+                name_text = _("Page Scroll Amount"),
+                toggle = { C_("Scroll", "legacy"), C_("Scroll", "small"), C_("Scroll", "medium"), C_("Scroll", "large") },
+                values = { 0, 1, 2, 3 },
+                default_value = 0,
+                args = { 0, 1, 2, 3 },
+                event = "PageScrollAmountUpdate",
+                enabled_func = function(configurable)
+                    return optionsutil.enableIfEquals(configurable, "page_scroll", 1)
+                end,
+                name_text_hold_callback = optionsutil.showValues,
+                help_text = _([[In continuous view mode, sets the amount of scroll tapping would lead to.]]),
+            },
             {
                 name = "full_screen",
                 name_text = _("Progress Bar"),