Make alignment a widget property

src/widget/position/scroll.rs

@@ -14,13 +14,9 @@ impl Widget for Scroll {
         let container_len = painter.px_len(self.axis);
         // Draw in the whole container only when its scrolling-axis length is
         // not already known, then draw it at the scrolled offset.
-        let (answer_len, measured) = match painter.known_len(&self.inner, self.axis, UiRegion::FULL)
-        {
-            Some(len) => (len, None),
-            None => {
-                let size = painter.widget_within(&self.inner, UiRegion::FULL).size();
-                (size.axis(self.axis), Some(size))
-            }
+        let answer_len = match painter.known_len(&self.inner, self.axis, UiRegion::FULL) {
+            Some(len) => len,
+            None => painter.widget(&self.inner).size().axis(self.axis),
         };
         let content = answer_len.apply_leftover();
         self.container_len = container_len;
@@ -30,21 +26,33 @@ impl Widget for Scroll {
             self.amt = self.content_len - self.container_len;
         }
         self.update_amt();
-        // Content of a fixed length that fits sits at the start of any box
-        // it fits in; one scrolled part way sits where it is until the box
-        // shrinks past what is left of it. Kept to the end, it moves with
-        // every length.
-        if content.rel == 0.0 && self.content_len <= self.container_len {
+        let align = painter.alignment().axis(self.axis);
+        // Content of a fixed length that fits sits at the start of any box it
+        // fits in -- but only anchored there. Anywhere else it is a part of
+        // the room left over, so it moves with every length the box takes and
+        // the drawing holds for that length alone. One scrolled part way sits
+        // where it is until the box shrinks past what is left of it. Kept to
+        // the end, it moves with every length.
+        if content.rel == 0.0 && self.content_len <= self.container_len && align == AxisAlign::NEG {
             painter.holds(self.axis, self.content_len..=f32::INFINITY);
         } else if content.rel == 0.0 && !self.snap_end {
             let left = self.content_len - self.amt;
             painter.holds(self.axis, f32::NEG_INFINITY..=left);
         }
 
-        let mut region = UiRegion::FULL.offset(Vec2::from_axis(self.axis, -self.amt, 0.0));
+        // Content shorter than the viewport has room to sit in, and where it
+        // sits is this widget's own alignment -- the same property that would
+        // have placed the whole scroll in a box longer than it.
+        let slack = (self.container_len - self.content_len).max(0.0);
+        let anchor = slack * align.rel();
+        let mut region = UiRegion::FULL.offset(Vec2::from_axis(self.axis, anchor - self.amt, 0.0));
         region.axis_mut(self.axis).end = region.axis(self.axis).start.offset(self.content_len);
-        let placed = painter.widget_within(&self.inner, region).size();
-        measured.unwrap_or_else(|| Size::from_axis(self.axis, answer_len, placed.axis(!self.axis)))
+        painter.widget_aligned(&self.inner, region, RegionAlign::NEAR);
+        // What it occupies is its box, on both axes: it clips its content to
+        // that box, so it can neither take less of one nor honestly ask for
+        // more. The content's length is what it scrolls through, not what it
+        // is.
+        Size::LEFTOVER
     }
 }