ui: chrome live_move for title-bar moves

Moves a rect with a title-bar drag from the rect and press position kept at
drag start, so the window follows the pointer without drift. Escape or window
focus loss ends the move and returns the start rect.

crates/ui/src/docking/chrome.rs

@@ -3,7 +3,7 @@
 //! these; other hosts (task panes) use them with their own ids and actions. Nothing here changes
 //! a layout: the caller turns the responses into its own commands.
 
-use egui::{Color32, Context, FontId, Id, Key, LayerId, Order, Rect, Response, Sense, Stroke, StrokeKind, Ui, Vec2, WidgetText, pos2, vec2};
+use egui::{Color32, Context, FontId, Id, Key, LayerId, Order, Pos2, Rect, Response, Sense, Stroke, StrokeKind, Ui, Vec2, WidgetText, pos2, vec2};
 
 use super::{DockStyle, PanelLimits, resize::AnchoredResize};
 
@@ -345,3 +345,45 @@ impl<'a> TitleBar<'a> {
         TitleResponse { bar, menu, close }
     }
 }
+
+/// One step of a title-bar move, from [`live_move`].
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum MoveStep {
+    /// The rect follows the pointer, inside the bounds.
+    Moving(Rect),
+    /// Escape or window focus loss ended the move: the rect it started from.
+    Cancelled(Rect),
+    /// Released over `pointer`, with the rect where it was dropped.
+    Dropped { rect: Rect, pointer: Pos2 },
+}
+
+/// Move `rect` with a drag on `bar` (usually [`TitleResponse::bar`]). The rect and the press
+/// position at drag start are kept in egui memory under the bar's id, so the window follows the
+/// pointer without drift. Escape or window focus loss ends the move and returns the start rect;
+/// the rest of that drag is ignored. `None` when the bar is not being moved.
+pub fn live_move(ctx: &Context, bar: &Response, rect: Rect, bounds: Rect) -> Option<MoveStep> {
+    let key = bar.id.with("live-move");
+    if bar.drag_started()
+        && let Some(origin) = ctx.input(|input| input.pointer.press_origin())
+    {
+        ctx.data_mut(|data| data.insert_temp(key, (rect, origin)));
+    }
+    let (start, origin) = ctx.data(|data| data.get_temp::<(Rect, Pos2)>(key))?;
+    let forget = || ctx.data_mut(|data| data.remove::<(Rect, Pos2)>(key));
+    if ctx.input(|input| input.key_pressed(Key::Escape) || !input.focused) {
+        forget();
+        return Some(MoveStep::Cancelled(start));
+    }
+    let pointer = ctx.pointer_latest_pos().unwrap_or(origin);
+    let moved = clamp(start.translate(pointer - origin), bounds);
+    if bar.drag_stopped() {
+        forget();
+        return Some(MoveStep::Dropped { rect: moved, pointer });
+    }
+    if bar.dragged() {
+        return Some(MoveStep::Moving(moved));
+    }
+    // The drag ended without a release on this bar (the bar was not shown).
+    forget();
+    None
+}