Fix Ui::scroll_with_delta only scrolling if the ScrollArea is focused (#4303)

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This introduces the boolean field force_current_scroll_area to
InputState which will be set when scroll_with_delta is called, causing
the ScrollArea to skip the check whether it is focused and always
consume the smooth scroll delta.

* Closes #2783 
* Related to #4295

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/egui_demo_lib/src/demo/scrolling.rs

@@ -236,6 +236,7 @@ struct ScrollTo {
     track_item: usize,
     tack_item_align: Option<Align>,
     offset: f32,
+    delta: f32,
 }
 
 impl Default for ScrollTo {
@@ -244,6 +245,7 @@ impl Default for ScrollTo {
             track_item: 25,
             tack_item_align: Some(Align::Center),
             offset: 0.0,
+            delta: 64.0,
         }
     }
 }
@@ -258,6 +260,7 @@ impl super::View for ScrollTo {
         let mut go_to_scroll_offset = false;
         let mut scroll_top = false;
         let mut scroll_bottom = false;
+        let mut scroll_delta = None;
 
         ui.horizontal(|ui| {
             ui.label("Scroll to a specific item index:");
@@ -294,6 +297,20 @@ impl super::View for ScrollTo {
             scroll_bottom |= ui.button("Scroll to bottom").clicked();
         });
 
+        ui.horizontal(|ui| {
+            ui.label("Scroll by");
+            DragValue::new(&mut self.delta)
+                .speed(1.0)
+                .suffix("px")
+                .ui(ui);
+            if ui.button("⬇").clicked() {
+                scroll_delta = Some(self.delta * Vec2::UP); // scroll down (move contents up)
+            }
+            if ui.button("⬆").clicked() {
+                scroll_delta = Some(self.delta * Vec2::DOWN); // scroll up (move contents down)
+            }
+        });
+
         let mut scroll_area = ScrollArea::vertical().max_height(200.0).auto_shrink(false);
         if go_to_scroll_offset {
             scroll_area = scroll_area.vertical_scroll_offset(self.offset);
@@ -305,6 +322,10 @@ impl super::View for ScrollTo {
                 if scroll_top {
                     ui.scroll_to_cursor(Some(Align::TOP));
                 }
+                if let Some(scroll_delta) = scroll_delta {
+                    ui.scroll_with_delta(scroll_delta);
+                }
+
                 ui.vertical(|ui| {
                     for item in 1..=num_items {
                         if track_item && item == self.track_item {