Viewer: In flow mode, implement drag scrolling. Fixes #1880707 [there is no auto scroll when to try text select](https://bugs.launchpad.net/calibre/+bug/1880707)

src/pyj/read_book/flow_mode.pyj

@@ -254,6 +254,7 @@ class ScrollAnimator:
         return self.is_running() and (self.auto or self.auto_timer is not None)
 
     def start(self, direction, auto):
+        cancel_drag_scroll()
         if self.wait:
             return
 
@@ -274,6 +275,9 @@ class ScrollAnimator:
 
     def smooth_scroll(self, ts):
         duration = self.end_time - self.start_time
+        if duration <= 0:
+            self.animation_id = None
+            return
         progress = max(0, min(1, (ts - self.start_time) / duration)) # max/min to account for jitter
         scroll_target = self.start_offset
         scroll_target += Math.trunc(self.direction * progress * duration * line_height() * opts.lines_per_sec_smooth) / 1000
@@ -374,6 +378,7 @@ class FlickAnimator:
         self.animation_id = None
 
     def start(self, gesture):
+        cancel_drag_scroll()
         self.vertical = gesture.axis is 'vertical'
         now = window.performance.now()
         points = times = None
@@ -411,6 +416,70 @@ class FlickAnimator:
 
 flick_animator = FlickAnimator()
 
+
+class DragScroller:
+
+    DURATION = 100 # milliseconds
+
+    def __init__(self):
+        self.animation_id = None
+        self.direction = 1
+        self.speed_factor = 1
+        self.start_time = self.end_time = 0
+        self.start_offset = 0
+
+    def is_running(self):
+        return self.animation_id is not None
+
+    def smooth_scroll(self, ts):
+        duration = self.end_time - self.start_time
+        if duration <= 0:
+            self.animation_id = None
+            self.start(self.direction, self.speed_factor)
+            return
+        progress = max(0, min(1, (ts - self.start_time) / duration)) # max/min to account for jitter
+        scroll_target = self.start_offset
+        scroll_target += Math.trunc(self.direction * progress * duration * line_height() * opts.lines_per_sec_smooth * self.speed_factor) / 1000
+        window.scrollTo(0, scroll_target)
+
+        if progress < 1:
+            self.animation_id = window.requestAnimationFrame(self.smooth_scroll)
+        else:
+            self.animation_id = None
+            self.start(self.direction, self.speed_factor)
+
+    def start(self, direction, speed_factor):
+        now = window.performance.now()
+        self.end_time = now + self.DURATION
+        if not self.is_running() or direction is not self.direction or speed_factor is not self.speed_factor:
+            self.stop()
+            self.direction = direction
+            self.speed_factor = speed_factor
+            self.start_time = now
+            self.start_offset = window.pageYOffset
+            self.animation_id = window.requestAnimationFrame(self.smooth_scroll)
+
+    def stop(self):
+        if self.animation_id is not None:
+            window.cancelAnimationFrame(self.animation_id)
+            self.animation_id = None
+
+
+drag_scroller = DragScroller()
+
+
+def cancel_drag_scroll():
+    drag_scroller.stop()
+
+
+def start_drag_scroll(delta):
+    limit = opts.margin_top if delta < 0 else opts.margin_bottom
+    direction = 1 if delta >= 0 else -1
+    speed_factor = min(abs(delta), limit) / limit
+    speed_factor *= (2 - speed_factor)  # QuadOut Easing curve
+    drag_scroller.start(direction, 2 * speed_factor)
+
+
 def handle_gesture(gesture):
     flick_animator.stop()
     if gesture.type is 'swipe':

src/pyj/read_book/iframe.pyj

@@ -22,10 +22,11 @@ from read_book.extract import get_elements
 from read_book.find import reset_find_caches, select_search_result
 from read_book.flow_mode import (
     anchor_funcs as flow_anchor_funcs, auto_scroll_action as flow_auto_scroll_action,
-    ensure_selection_visible, flow_onwheel, flow_to_scroll_fraction,
-    handle_gesture as flow_handle_gesture, handle_shortcut as flow_handle_shortcut,
-    layout as flow_layout, scroll_by_page as flow_scroll_by_page,
-    scroll_to_extend_annotation as flow_annotation_scroll
+    cancel_drag_scroll, ensure_selection_visible, flow_onwheel,
+    flow_to_scroll_fraction, handle_gesture as flow_handle_gesture,
+    handle_shortcut as flow_handle_shortcut, layout as flow_layout,
+    scroll_by_page as flow_scroll_by_page,
+    scroll_to_extend_annotation as flow_annotation_scroll, start_drag_scroll
 )
 from read_book.footnotes import is_footnote_link
 from read_book.globals import (
@@ -143,6 +144,7 @@ class IframeBoss:
         self.length_before = None
 
     def on_overlay_visibility_changed(self, data):
+        cancel_drag_scroll()
         if data.visible:
             self.forward_keypresses = True
             if self.auto_scroll_action:
@@ -166,6 +168,8 @@ class IframeBoss:
         window.addEventListener('resize', debounce(self.onresize, 500))
         window.addEventListener('wheel', self.onwheel, {'passive': False})
         window.addEventListener('keydown', self.onkeydown, {'passive': False})
+        window.addEventListener('mousemove', self.onmousemove, {'passive': True})
+        window.addEventListener('mouseup', self.onmouseup, {'passive': True})
         document.documentElement.addEventListener('contextmenu', self.oncontextmenu, {'passive': False})
         document.addEventListener('selectionchange', self.onselectionchange)
         self.color_scheme = data.color_scheme
@@ -201,6 +205,7 @@ class IframeBoss:
             (console.error or console.log)('There was an error in the JavaScript from within the book')
 
     def display(self, data):
+        cancel_drag_scroll()
         self.length_before = None
         self.content_ready = False
         clear_annot_id_uuid_map()
@@ -535,6 +540,22 @@ class IframeBoss:
             else:
                 self.handle_wheel(evt)
 
+    def onmousemove(self, evt):
+        if evt.buttons is not 1:
+            return
+        if 0 <= evt.clientY <= window.innerHeight or current_layout_mode() is not 'flow':
+            cancel_drag_scroll()
+            return
+        sel = window.getSelection()
+        if not sel:
+            cancel_drag_scroll()
+            return
+        delta = evt.clientY if evt.clientY < 0 else (evt.clientY - window.innerHeight)
+        start_drag_scroll(delta)
+
+    def onmouseup(self, evt):
+        cancel_drag_scroll()
+
     def onkeydown(self, evt):
         if current_layout_mode() is not 'flow' and evt.key is 'Tab':
             # Prevent the TAB key from shifting focus as it causes partial scrolling

src/pyj/read_book/settings.pyj

@@ -22,6 +22,8 @@ def update_settings(settings):
     opts.lines_per_sec_smooth = settings.lines_per_sec_smooth
     opts.margin_left = max(0, settings.margin_left)
     opts.margin_right = max(0, settings.margin_right)
+    opts.margin_top = max(0, settings.margin_top)
+    opts.margin_bottom = max(0, settings.margin_bottom)
     opts.override_book_colors = settings.override_book_colors
     opts.paged_wheel_scrolls_by_screen = v'!!settings.paged_wheel_scrolls_by_screen'
     opts.paged_taps_scroll_by_screen = v'!!settings.paged_taps_scroll_by_screen'

src/pyj/read_book/view.pyj

@@ -840,6 +840,8 @@ class View:
         return {
             'margin_left': 0 if name is self.book.manifest.title_page_name else sd.get('margin_left'),
             'margin_right': 0 if name is self.book.manifest.title_page_name else sd.get('margin_right'),
+            'margin_top': 0 if name is self.book.manifest.title_page_name else sd.get('margin_top'),
+            'margin_bottom': 0 if name is self.book.manifest.title_page_name else sd.get('margin_bottom'),
             'read_mode': sd.get('read_mode'),
             'columns_per_screen': sd.get('columns_per_screen'),
             'color_scheme': cs,