Allow scroll into view without specifying an alignment (#1247)

* Allow scroll into view without specifying an alignment
* Handle case of UI being too big to fit in the scroll view

CHANGELOG.md

@@ -25,6 +25,7 @@ NOTE: [`epaint`](epaint/CHANGELOG.md), [`eframe`](eframe/CHANGELOG.md), [`egui_w
 * Added `Response::on_hover_text_at_pointer` as a convenience akin to `Response::on_hover_text` ([1179](https://github.com/emilk/egui/pull/1179)).
 * Added `ui.weak(text)`.
 * Added `Slider::step_by` ([1255](https://github.com/emilk/egui/pull/1225)).
+* Added ability to scroll an UI into view without specifying an alignment ([1247](https://github.com/emilk/egui/pull/1247))
 
 ### Changed 🔧
 * ⚠️ `Context::input` and `Ui::input` now locks a mutex. This can lead to a dead-lock is used in an `if let` binding!

egui/src/containers/scroll_area.rs

@@ -489,18 +489,37 @@ impl Prepared {
                 // We take the scroll target so only this ScrollArea will use it:
                 let scroll_target = content_ui.ctx().frame_state().scroll_target[d].take();
                 if let Some((scroll, align)) = scroll_target {
-                    let center_factor = align.to_factor();
-
                     let min = content_ui.min_rect().min[d];
-                    let visible_range = min..=min + content_ui.clip_rect().size()[d];
+                    let clip_rect = content_ui.clip_rect();
-                    let offset = scroll - lerp(visible_range, center_factor);
+                    let visible_range = min..=min + clip_rect.size()[d];
-
+                    let start = *scroll.start();
+                    let end = *scroll.end();
+                    let clip_start = clip_rect.min[d];
+                    let clip_end = clip_rect.max[d];
                     let mut spacing = ui.spacing().item_spacing[d];
 
-                    // Depending on the alignment we need to add or subtract the spacing
+                    if let Some(align) = align {
-                    spacing *= remap(center_factor, 0.0..=1.0, -1.0..=1.0);
+                        let center_factor = align.to_factor();
 
-                    state.offset[d] = offset + spacing;
+                        let offset =
+                            lerp(scroll, center_factor) - lerp(visible_range, center_factor);
+
+                        // Depending on the alignment we need to add or subtract the spacing
+                        spacing *= remap(center_factor, 0.0..=1.0, -1.0..=1.0);
+
+                        state.offset[d] = offset + spacing;
+                    } else if start < clip_start && end < clip_end {
+                        let min_adjust =
+                            (clip_start - start + spacing).min(clip_end - end - spacing);
+                        state.offset[d] -= min_adjust;
+                    } else if end > clip_end && start > clip_start {
+                        let min_adjust =
+                            (end - clip_end + spacing).min(start - clip_start - spacing);
+                        state.offset[d] += min_adjust;
+                    } else {
+                        // Ui is already in view, no need to adjust scroll.
+                        continue;
+                    };
                 }
             }
         }

egui/src/frame_state.rs

@@ -1,3 +1,5 @@
+use std::ops::RangeInclusive;
+
 use crate::*;
 
 /// State that is collected during a frame and then cleared.
@@ -28,7 +30,7 @@ pub(crate) struct FrameState {
     /// Cleared by the first `ScrollArea` that makes use of it.
     pub(crate) scroll_delta: Vec2, // TODO: move to a Mutex inside of `InputState` ?
     /// horizontal, vertical
-    pub(crate) scroll_target: [Option<(f32, Align)>; 2],
+    pub(crate) scroll_target: [Option<(RangeInclusive<f32>, Option<Align>)>; 2],
 }
 
 impl Default for FrameState {
@@ -40,7 +42,7 @@ impl Default for FrameState {
             used_by_panels: Rect::NAN,
             tooltip_rect: None,
             scroll_delta: Vec2::ZERO,
-            scroll_target: [None; 2],
+            scroll_target: [None, None],
         }
     }
 }
@@ -63,7 +65,7 @@ impl FrameState {
         *used_by_panels = Rect::NOTHING;
         *tooltip_rect = None;
         *scroll_delta = input.scroll_delta;
-        *scroll_target = [None; 2];
+        *scroll_target = [None, None];
     }
 
     /// How much space is still available after panels has been added.

egui/src/response.rs

@@ -1,5 +1,5 @@
 use crate::{
-    emath::{lerp, Align, Pos2, Rect, Vec2},
+    emath::{Align, Pos2, Rect, Vec2},
     menu, Context, CursorIcon, Id, LayerId, PointerButton, Sense, Ui, WidgetText,
     NUM_POINTER_BUTTONS,
 };
@@ -443,7 +443,10 @@ impl Response {
         )
     }
 
-    /// Move the scroll to this UI with the specified alignment.
+    /// Adjust the scroll position until this UI becomes visible. If `align` is not provided, it'll scroll enough to
+    /// bring the UI into view.
+    ///
+    /// See also [`Ui::scroll_to_cursor`]
     ///
     /// ```
     /// # egui::__run_test_ui(|ui| {
@@ -451,18 +454,15 @@ impl Response {
     ///     for i in 0..1000 {
     ///         let response = ui.button("Scroll to me");
     ///         if response.clicked() {
-    ///             response.scroll_to_me(egui::Align::Center);
+    ///             response.scroll_to_me(Some(egui::Align::Center));
     ///         }
     ///     }
     /// });
     /// # });
     /// ```
-    pub fn scroll_to_me(&self, align: Align) {
+    pub fn scroll_to_me(&self, align: Option<Align>) {
-        let scroll_target = lerp(self.rect.x_range(), align.to_factor());
+        self.ctx.frame_state().scroll_target[0] = Some((self.rect.x_range(), align));
-        self.ctx.frame_state().scroll_target[0] = Some((scroll_target, align));
+        self.ctx.frame_state().scroll_target[1] = Some((self.rect.y_range(), align));
-
-        let scroll_target = lerp(self.rect.y_range(), align.to_factor());
-        self.ctx.frame_state().scroll_target[1] = Some((scroll_target, align));
     }
 
     /// For accessibility.

egui/src/ui.rs

@@ -889,7 +889,10 @@ impl Ui {
         (response, painter)
     }
 
-    /// Move the scroll to this cursor position with the specified alignment.
+    /// Adjust the scroll position until the cursor becomes visible. If `align` is not provided, it'll scroll enough to
+    /// bring the cursor into view.
+    ///
+    /// See also [`Response::scroll_to_me`]
     ///
     /// ```
     /// # use egui::Align;
@@ -901,15 +904,16 @@ impl Ui {
     ///     }
     ///
     ///     if scroll_bottom {
-    ///         ui.scroll_to_cursor(Align::BOTTOM);
+    ///         ui.scroll_to_cursor(Some(Align::BOTTOM));
     ///     }
     /// });
     /// # });
     /// ```
-    pub fn scroll_to_cursor(&mut self, align: Align) {
+    pub fn scroll_to_cursor(&mut self, align: Option<Align>) {
         let target = self.next_widget_position();
         for d in 0..2 {
-            self.ctx().frame_state().scroll_target[d] = Some((target[d], align));
+            let target = target[d];
+            self.ctx().frame_state().scroll_target[d] = Some((target..=target, align));
         }
     }
 }

egui_demo_lib/src/apps/demo/scrolling.rs

@@ -147,7 +147,7 @@ fn huge_content_painter(ui: &mut egui::Ui) {
 #[derive(PartialEq)]
 struct ScrollTo {
     track_item: usize,
-    tack_item_align: Align,
+    tack_item_align: Option<Align>,
     offset: f32,
 }
 
@@ -155,7 +155,7 @@ impl Default for ScrollTo {
     fn default() -> Self {
         Self {
             track_item: 25,
-            tack_item_align: Align::Center,
+            tack_item_align: Some(Align::Center),
             offset: 0.0,
         }
     }
@@ -180,13 +180,16 @@ impl super::View for ScrollTo {
         ui.horizontal(|ui| {
             ui.label("Item align:");
             track_item |= ui
-                .radio_value(&mut self.tack_item_align, Align::Min, "Top")
+                .radio_value(&mut self.tack_item_align, Some(Align::Min), "Top")
                 .clicked();
             track_item |= ui
-                .radio_value(&mut self.tack_item_align, Align::Center, "Center")
+                .radio_value(&mut self.tack_item_align, Some(Align::Center), "Center")
                 .clicked();
             track_item |= ui
-                .radio_value(&mut self.tack_item_align, Align::Max, "Bottom")
+                .radio_value(&mut self.tack_item_align, Some(Align::Max), "Bottom")
+                .clicked();
+            track_item |= ui
+                .radio_value(&mut self.tack_item_align, None, "None (Bring into view)")
                 .clicked();
         });
 
@@ -213,7 +216,7 @@ impl super::View for ScrollTo {
         let (current_scroll, max_scroll) = scroll_area
             .show(ui, |ui| {
                 if scroll_top {
-                    ui.scroll_to_cursor(Align::TOP);
+                    ui.scroll_to_cursor(Some(Align::TOP));
                 }
                 ui.vertical(|ui| {
                     for item in 1..=50 {
@@ -228,7 +231,7 @@ impl super::View for ScrollTo {
                 });
 
                 if scroll_bottom {
-                    ui.scroll_to_cursor(Align::BOTTOM);
+                    ui.scroll_to_cursor(Some(Align::BOTTOM));
                 }
 
                 let margin = ui.visuals().clip_rect_margin;