feat(gui-1b): B2's horizontal leg --- the panel origin actually moves

`PKind::ScrollLeft | PKind::ScrollRight` were CLAIMED AND DROPPED in the
panel replay, with a comment assigning the axis to Stage 1b. That is the
"frontend emits, receiver discards" shape the panel-replay lane was
opened to fix, inherited for the horizontal axis. This closes it.

`scroll_window_columns` moves the side window's `view_left` by B7's
bound, stated exactly: `0 ..= widest - viewport`, saturating at zero, so
the final display column stays visible --- clamping at the widest line's
full width would let the origin pass every glyph and blank the viewport.
Wrap pins the origin to zero, matching `horizontal_follow`. It returns
whether the origin actually moved, which is lifetime clause 2's
"effective move".

The widest-line rule is SHARED. `display_width::widest_line_columns`
lives beside the module's other column helpers and both frontends use
it, for the same reason `scroll::follow_left` is shared: two frontends
that compute the right bound differently disagree about where the
document ends.

B2's row asserts the EFFECT --- `view_left` before and after --- not an
emission, and it carries the discriminating setup the bound requires: a
panel whose content fits has a maximum origin of zero, so the move is
absorbed by the clamp and a dropped event reads identical to correct
behaviour. The fixture gets a line wider than the viewport. Mutation:
restore the claimed-and-dropped arm, and the row fires.

Two mistakes of mine in this commit's history, both caught before it:

- I reverted a mutation with `git checkout -- src/editor.rs` on a file
  holding UNCOMMITTED work, and destroyed the whole B2 implementation.
  Re-applied, and the mutation check redone against a file snapshot ---
  the discipline I had used earlier in the CRDT lane and dropped here.
- Inserting the new test above an existing one STOLE ITS `#[test]` and
  its doc comment, so `r4_p1_a_chrome_press_neither_arms_nor_moves_point`
  silently stopped being a test. Clippy's "never used" caught it. Both
  are restored, and the suite count confirms 1994 tests rather than
  1993.

pmacs-gpu/src/main.rs

@@ -8377,6 +8377,17 @@ impl State {
     /// budget rows and is recorded rather than pre-optimised: a cache
     /// needs an invalidation key, and the wrong key is a worse defect
     /// than a measurable scan.
+    ///
+    /// **Which "display line" this measures is a boundary B3's witness
+    /// must settle.** This counts SOURCE-TEXT display columns — tab
+    /// stops and Unicode width — and therefore excludes rendered
+    /// projections such as inline adornments and math substitutions,
+    /// which can occupy a different width on screen than the bytes they
+    /// stand for. That is consistent with the TUI-derived column rule
+    /// the two frontends share, and it is a choice, not an oversight:
+    /// "widest display line" is readable the other way. Recorded here
+    /// so the witness states which meaning governs rather than
+    /// discovering it.
     fn widest_display_columns(&self) -> u32 {
         let widest = self
             .current_text

src/daemon.rs

@@ -7673,6 +7673,70 @@ mod tests {
 
     /// P1 — a press on the band's MODE LINE begins nothing.
     ///
+    /// GUI Stage 1b B2 — a horizontal panel notch reaches the daemon's
+    /// window-targeted `view_left` path and MOVES it.
+    ///
+    /// This arm used to be claimed and dropped, which is the
+    /// "frontend emits, receiver discards" shape §2a named. The row
+    /// asserts the **effect**, not the emission: `view_left` before and
+    /// after.
+    #[test]
+    fn b2_a_horizontal_panel_notch_moves_the_side_windows_view_left() {
+        let fid = FrontendId(791);
+        let (mut editor, mut states, mut render, _document, panel, epochs) =
+            panel_session_at(PROTOCOL_VERSION, fid);
+        // A CONTENT cell, not chrome: row 0 is inside the grid.
+        let (buffer_id, cell) = {
+            let core = editor.core.borrow();
+            (
+                core.windows[&panel].buffer_id,
+                pmacs_protocol::CellCoord::new(0, 0),
+            )
+        };
+        // **The discriminating setup.** B7's bound is
+        // `widest − viewport`, so a panel whose content fits has a
+        // maximum origin of zero and the move is absorbed by the
+        // clamp — correct behaviour that would read here as a dropped
+        // event. The row needs content wider than the viewport.
+        foreign_edit(&editor, buffer_id, "x".repeat(400).as_bytes());
+        let before = editor
+            .core
+            .borrow()
+            .windows
+            .get(&panel)
+            .map_or(0, |w| w.view_left);
+
+        let generation = live_generation(PanelArm::Mapped, &editor, &mut states, fid);
+        dispatch_panel_event(
+            &mut editor,
+            fid,
+            PROTOCOL_VERSION,
+            &mut states,
+            &mut render,
+            arm_pointer(
+                PanelArm::Mapped,
+                fid,
+                epochs,
+                buffer_id,
+                generation,
+                cell,
+                pmacs_protocol::MouseKind::ScrollRight,
+            ),
+        );
+
+        let after = editor
+            .core
+            .borrow()
+            .windows
+            .get(&panel)
+            .map_or(0, |w| w.view_left);
+        assert_ne!(
+            after, before,
+            "a horizontal panel notch must move the side window's origin, \
+             not be claimed and dropped"
+        );
+    }
+
     /// The merge made this arm the latch, because `Consumed` and
     /// `Accepted` were the same `true`. The row reads the cursor as well
     /// as the latch: a chrome press must not move point either.

src/display_width.rs

@@ -23,6 +23,24 @@ pub fn advance_char(column: u32, ch: char) -> u32 {
     column.saturating_add(width)
 }
 
+/// Widest line in `text`, in display columns.
+///
+/// GUI Stage 1b B7's upper-bound input, shared by both frontends so
+/// they cannot disagree about where the right bound is — the same
+/// reason `scroll::follow_left` is shared.
+///
+/// **This measures SOURCE-TEXT display columns**: tab stops and Unicode
+/// width. Rendered projections — inline adornments, math substitutions
+/// — can occupy a different width on screen and are deliberately not
+/// counted, matching the column rule the rest of this module states.
+#[must_use]
+pub fn widest_line_columns(text: &str) -> u32 {
+    text.split('\n')
+        .map(|line| line.chars().fold(0, advance_char))
+        .max()
+        .unwrap_or(0)
+}
+
 /// Display width of the valid UTF-8 prefix of `bytes`.
 ///
 /// Invalid input is conservatively truncated at the first invalid byte. This

src/editor.rs

@@ -3356,23 +3356,81 @@ impl EditorState {
                 }
                 self.open_context_menu(side, coord.row, coord.col, (coord.row, coord.col));
             }
-            // Claimed and dropped, for two different reasons kept in one
-            // arm because their bodies are identical: horizontal panel
-            // scrolling belongs to GUI arc Stage 1b's B-rows rather than
-            // parent 48, bare `Move` neither focuses nor claims, and the
-            // remaining buttons have no panel semantics at all.
-            PKind::ScrollLeft
-            | PKind::ScrollRight
-            | PKind::Move
-            | PKind::Down(_)
-            | PKind::Up(_)
-            | PKind::Drag(_) => {}
+            // GUI Stage 1b B2 — the horizontal leg, which this arm used
+            // to claim and drop. One notch is `SCROLL_COLUMNS`; the
+            // frontend banks fractions and sends whole notches, so the
+            // step is applied here exactly once.
+            PKind::ScrollLeft => {
+                self.scroll_window_columns(side, -SCROLL_COLUMNS);
+            }
+            PKind::ScrollRight => {
+                self.scroll_window_columns(side, SCROLL_COLUMNS);
+            }
+            // Bare `Move` neither focuses nor claims, and the remaining
+            // buttons have no panel semantics at all.
+            PKind::Move | PKind::Down(_) | PKind::Up(_) | PKind::Drag(_) => {}
         }
         // Only a left press can anchor, and it returns `true` above.
         // Every other kind reaching here handled something already live.
         false
     }
 
+    /// GUI Stage 1b B2/B3/B7 — move one window's horizontal origin by
+    /// `columns`, the daemon-side effect a panel-document horizontal
+    /// wheel reaches.
+    ///
+    /// This closes the leg §2a named: `ScrollLeft`/`ScrollRight` were
+    /// **claimed and dropped** here, which is the "frontend emits,
+    /// receiver discards" shape the panel-replay lane was opened to fix
+    /// and that B1–B3 inherited for the horizontal axis.
+    ///
+    /// The bound is B7's, stated exactly — `0 ..= widest − viewport`,
+    /// **saturating at zero** — so the final display column stays
+    /// visible; clamping at the widest line's full width would let the
+    /// origin pass every glyph and blank the viewport. **Wrap pins the
+    /// origin to zero**, matching `horizontal_follow`.
+    ///
+    /// Returns whether the origin actually moved, which is clause 2's
+    /// "effective move".
+    fn scroll_window_columns(&mut self, win_id: WindowId, columns: i32) -> bool {
+        let mut core = self.core.borrow_mut();
+        let Some(window) = core.windows.get(&win_id) else {
+            return false;
+        };
+        if window.last_wrap == crate::view::WrapMode::Wrap {
+            if let Some(window) = core.windows.get_mut(&win_id) {
+                window.view_left = 0;
+            }
+            return false;
+        }
+        let buffer_id = window.buffer_id;
+        let viewport_cols = window.last_content_cols;
+        let old_left = window.view_left;
+        let registry = core.registry.clone();
+        let widest = {
+            let reg = registry.borrow();
+            let Ok(buf) = reg.get(buffer_id) else {
+                return false;
+            };
+            let len = buf.len();
+            let mut bytes = vec![0u8; len as usize];
+            buf.snapshot_rope().slice(0, len, &mut bytes);
+            crate::display_width::widest_line_columns(&String::from_utf8_lossy(&bytes))
+        };
+        let max_left = widest.saturating_sub(viewport_cols);
+        let next = i64::from(old_left)
+            .saturating_add(i64::from(columns))
+            .clamp(0, i64::from(max_left));
+        let next = u32::try_from(next).unwrap_or(0);
+        if next == old_left {
+            return false;
+        }
+        if let Some(window) = core.windows.get_mut(&win_id) {
+            window.view_left = next;
+        }
+        true
+    }
+
     /// Byte under a panel cell, resolved against the SIDE window's own
     /// `view_top` and fold map.
     ///
@@ -4654,6 +4712,11 @@ impl EditorState {
 /// readline / Emacs default and is what most terminal users expect.
 const SCROLL_LINES: i32 = 3;
 
+/// Columns one horizontal wheel notch moves a window's origin — B7's
+/// "three columns per wheel tick", the horizontal twin of
+/// [`SCROLL_LINES`].
+const SCROLL_COLUMNS: i32 = 3;
+
 /// Gutter marker drawn on a collapsed region's head row (Arc 6 Stage 2,
 /// Q#FD20). Occupies the gutter's leading pad cell — the same cell the
 /// diagnostic sign uses — so it adds no column and changes no width; it