Button: Better handling of translucent MovableContainer (#7223)

* DictQuickLookup: Preserve alpha when switching dict, and scrolling
inside a dict.

* Start moving the NumberPicker alpha hack to Button itself

This makes handling flash_ui easier and saner, avoiding flickering.

* Handle the transparency hack entirely from within Button

* Murder the now unnecessary NumberPicker update_callback hack

* Tweak comments

* And the Button handling made that redundant, too

* Squish debug print

* More comment tweaks

* Reset transparency on scrolling instead of rpeserving it

* Reset alpha when switching dictionaries

* Simplify the pre/post callbakc transparency state handling

And explain why we need to care.

* Give a named reference to ButtonDialog's MovableContainer, so the Button
alpha hack behaves with it

* Document the "self.movable" convention

* Amend that comment a bit

e.g., we don't care much about MultiConfirmBox'w MpvableContainer, as
any button action will close it.

* And make SkimTo's MovableContainer accessible so that Button can grok
that it's translucent

frontend/ui/widget/button.lua

@@ -234,6 +234,13 @@ function Button:showHide(show)
 end
 
 function Button:onTapSelectButton()
+    -- NOTE: We have a few tricks up our sleeve in case our parent is inside a translucent MovableContainer...
+    local was_translucent = self.show_parent and self.show_parent.movable and self.show_parent.movable.alpha
+    -- We make a distinction between transparency pre- and post- callback, because if a widget *was* transparent,
+    -- but no longer is post-callback, we want to ensure that we refresh the *full* container,
+    -- instead of just the button's frame, in order to avoid leaving bits of the widget transparent ;).
+    local is_translucent = was_translucent
+
     if self.enabled and self.callback then
         if G_reader_settings:isFalse("flash_ui") then
             self.callback()
@@ -276,7 +283,13 @@ function Button:onTapSelectButton()
                 UIManager:forceRePaint() -- Ensures we have a chance to see the highlight
             end
             self.callback()
-            UIManager:forceRePaint() -- Ensures whatever the callback wanted to paint will be shown *now*...
+            -- Check if the callback reset transparency...
+            is_translucent = was_translucent and self.show_parent.movable.alpha
+            -- We don't want to fence the callback when we're *still* translucent, because we want a *single* refresh post-callback *and* post-unhighlight,
+            -- in order to avoid flickering.
+            if not is_translucent then
+                UIManager:forceRePaint() -- Ensures whatever the callback wanted to paint will be shown *now*...
+            end
             if self.vsync then
                 -- NOTE: This is mainly useful when the callback caused a REAGL update that we do not explicitly fence already,
                 --       (i.e., Kobo Mk. 7).
@@ -346,6 +359,16 @@ function Button:onTapSelectButton()
     elseif type(self.tap_input_func) == "function" then
         self:onInput(self.tap_input_func())
     end
+
+    -- If our parent belongs to a translucent MovableContainer, repaint all the things to honor alpha without layering glitches,
+    -- and refresh the full container, because the widget might have inhibited its own setDirty call to avoid flickering (c.f., *SpinWidget).
+    if was_translucent then
+        -- If the callback reset the transparency, we only need to repaint our parent
+        UIManager:setDirty(is_translucent and "all" or self.show_parent, function()
+            return "ui", self.show_parent.movable.dimen
+        end)
+    end
+
     if self.readonly ~= true then
         return true
     end