Write a shared constant once, and stop a scroll placing its own content

Two findings from a sweep over the WGSL prelude and the position widgets,
scoped against upstream/main at ca2b4b2.

`module_source` already builds each shader's preamble from iris_core's own
constants, so the move-chain work's second copy of `MOVE_NONE` and
`CHAIN_LIMIT` -- under "keep in step with iris_core::CHAIN_LIMIT" -- asked a
reader by hand for what the mechanism beside it exists to do. Both are
injected now, with `MASK_NONE` beside them replacing a bare literal, and the
shader declares none of them.

`Scroll`'s `content_len` is never less than its box, so `slack` and the
`anchor` computed from it were always zero whatever the alignment: the
framework centres short content by placing the answer in the whole box, and
the comment credited arithmetic that could not have done it. The same belief
guarded the fits-in-the-box contract with `align == NEG`, so at the default
alignment -- the middle -- every box change redrew the scroll, measured as 1
widget against 0 at TOP_LEFT. `align` now has no reader at all.

`UiSpan::translated` and `UiRegion::translated` are reachable only from each
other and from nothing else.

Format, clippy with and without layout-diagnostics, and the 131-test suite
are clean. The cold dump over 400 depth-5 trees is byte-identical to
1096c31, and all three seed scans pass: 400 at depth 5 in 69.07s, 1,000 at
depth 6 in 169.29s, 2,000 at depth 4 in 300.75s.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/widget/position/scroll.rs

@@ -24,36 +24,36 @@ impl Widget for Scroll {
             self.amt = self.content_len - self.container_len;
         }
         self.update_amt();
-        let align = painter.alignment()[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.
+        // Reading the box in pixels above holds this drawing to that one
+        // length, so these two say where it holds more widely.
+        //
+        // Content of a fixed length that fits is handed the whole box below,
+        // and nothing here reads the box again, so every longer box gives the
+        // same drawing: it holds from the length the content needs upwards,
+        // and shrinking past that is what changes it. Where it sits in a box
+        // longer than itself is not this widget's to say -- placing its
+        // answer in the whole box is its own alignment, and that placement is
+        // a fraction of the box, so it holds at every length too.
+        //
+        // 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.
         let answer_is_px = answer_len.is_px();
-        if answer_is_px && self.content_len <= self.container_len && align == AxisAlign::NEG {
+        if answer_is_px && self.content_len <= self.container_len {
             painter.holds(self.axis, answer_px..=Px::MAX);
         } else if answer_is_px && !self.snap_end {
             let left = self.content_len - self.amt;
             painter.holds(self.axis, Px::MIN..=left);
         }
 
-        // 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(Px::ZERO);
-        let anchor = slack.mul(align.rel());
         // Content that fills the viewport and has not been scrolled is the
         // viewport, and is handed back as it came. Writing the same box as
         // its own length in pixels is the same box in another form, and the
         // two do not round alike: a part centred in `rel 1` lands a step from
         // one centred in `px 900`, since halving a difference is not halving
         // each part of it.
-        let moved = anchor != Px::ZERO || self.amt != Px::ZERO;
-        let content = match moved || self.content_len != self.container_len {
+        let content = match self.amt != Px::ZERO || self.content_len != self.container_len {
             true => {
-                let start = Len::from_parts(Rel::ZERO, anchor - self.amt);
+                let start = Len::from_parts(Rel::ZERO, -self.amt);
                 UiSpan::new(start, start.offset(self.content_len)).shifted_desc()
             }
             false => PlaceDescAxis::WHOLE,

src/widget/text/edit.rs

@@ -321,12 +321,10 @@ impl<'a> TextEditCtx<'a> {
         let old = (self.text.view.buf.text().to_string(), self.text.selection);
         let mut undo = false;
         let res = self.apply_event_inner(event, modifiers, &mut undo);
-        if undo {
-            if let Some((old, selection)) = self.text.history.pop() {
-                self.set(&old);
-                self.text.selection = selection;
-                self.clamp_selection_to_layout();
-            }
+        if undo && let Some((old, selection)) = self.text.history.pop() {
+            self.set(&old);
+            self.text.selection = selection;
+            self.clamp_selection_to_layout();
         } else if self.text.view.buf.text() != old.0 {
             self.text.history.push(old);
         }