feat: add marquee multi-select and always-on middle-button pan

- Drag on empty space in Select tool draws a marquee rectangle;
  elements intersecting it on release become selected
- Shift+click toggles elements in/out of the selection set
- Moving and deleting operate on all selected elements at once
- Middle mouse button drag now pans unconditionally in every tool mode
- Session selection state changed from Option<String> to Vec<String>
- New DragState::MarqueeSelecting variant with dashed rect rendering
- 29 tests passing, clippy clean

crates/agcanvas/src/app.rs

@@ -3,7 +3,7 @@ use crate::canvas::{CanvasInteraction, CanvasState};
 use crate::clipboard::ClipboardManager;
 use crate::command_palette::{CommandId, CommandPalette};
 use crate::drawing::{
-    draw_creation_preview, draw_elements, draw_selection, find_handle_at_screen_pos,
+    draw_creation_preview, draw_elements, draw_marquee, draw_selection, find_handle_at_screen_pos,
     screen_to_canvas, DragState, DrawingElement, Shape, ShapeStyle, Tool,
 };
 use crate::history::{ChangeSource, HistoryTree, NodeId};
@@ -347,9 +347,7 @@ impl AgCanvasApp {
                 DrawingCommand::Delete { session_id, id } => {
                     if let Some(session) = self.sessions.iter_mut().find(|s| s.id == session_id) {
                         session.drawing_elements.retain(|e| e.id != id);
-                        if session.selected_element_id.as_deref() == Some(&id) {
+                        session.selected_element_ids.retain(|selected_id| selected_id != &id);
-                            session.selected_element_id = None;
-                        }
                         session.record_edit(
                             "Agent: Delete Element",
                             ChangeSource::Agent { name: None },
@@ -359,7 +357,7 @@ impl AgCanvasApp {
                 DrawingCommand::Clear { session_id } => {
                     if let Some(session) = self.sessions.iter_mut().find(|s| s.id == session_id) {
                         session.drawing_elements.clear();
-                        session.selected_element_id = None;
+                        session.selected_element_ids.clear();
                         session
                             .record_edit("Agent: Clear Canvas", ChangeSource::Agent { name: None });
                     }
@@ -606,11 +604,19 @@ fn handle_select_tool(
     zoom: f32,
     pointer_pos: Option<egui::Pos2>,
 ) {
-    if response.drag_started() {
+    if response.drag_started_by(egui::PointerButton::Primary) {
         if let Some(screen_pos) = pointer_pos {
-            if let Some(selected_el) = session.selected_element() {
+            let shift_held = response.ctx.input(|i| i.modifiers.shift);
-                if let Some(handle) =
+
-                    find_handle_at_screen_pos(selected_el, screen_pos, canvas_center, offset, zoom)
+            if let Some((handle, selected_el)) = session
+                .drawing_elements
+                .iter()
+                .rev()
+                .filter(|el| session.selected_element_ids.contains(&el.id))
+                .find_map(|el| {
+                    find_handle_at_screen_pos(el, screen_pos, canvas_center, offset, zoom)
+                        .map(|handle| (handle, el))
+                })
             {
                 let original_rect = selected_el.bounding_rect();
                 let eid = selected_el.id.clone();
@@ -621,7 +627,6 @@ fn handle_select_tool(
                 };
                 return;
             }
-            }
 
             let canvas_pos = screen_to_canvas(screen_pos, canvas_center, offset, zoom);
             let hit = session
@@ -632,26 +637,50 @@ fn handle_select_tool(
 
             if let Some(el) = hit {
                 let eid = el.id.clone();
-                session.selected_element_id = Some(eid.clone());
+                if shift_held {
-                session.drag_state = DragState::Moving { element_id: eid };
+                    if session.selected_element_ids.contains(&eid) {
+                        session.selected_element_ids.retain(|id| id != &eid);
                     } else {
-                session.selected_element_id = None;
+                        session.selected_element_ids.push(eid);
+                    }
                     session.drag_state = DragState::None;
+                } else {
+                    if !session.selected_element_ids.contains(&eid) {
+                        session.selected_element_ids = vec![eid.clone()];
+                    }
+                    session.drag_state = DragState::Moving {
+                        element_ids: session.selected_element_ids.clone(),
+                    };
+                }
+            } else {
+                if !shift_held {
+                    session.selected_element_ids.clear();
+                }
+                session.drag_state = DragState::MarqueeSelecting {
+                    start: canvas_pos,
+                    current: canvas_pos,
+                };
             }
         }
     }
 
-    if response.dragged() {
+    if response.dragged_by(egui::PointerButton::Primary) {
         let delta_screen = response.drag_delta();
         let delta_canvas = delta_screen / zoom;
 
-        match &session.drag_state {
+        match &mut session.drag_state {
-            DragState::Moving { element_id, .. } => {
+            DragState::Moving { element_ids } => {
-                let eid = element_id.clone();
+                for eid in element_ids.iter() {
-                if let Some(el) = session.drawing_elements.iter_mut().find(|e| e.id == eid) {
+                    if let Some(el) = session.drawing_elements.iter_mut().find(|e| e.id == *eid) {
                         el.translate(delta_canvas);
                     }
                 }
+            }
+            DragState::MarqueeSelecting { current, .. } => {
+                if let Some(screen_pos) = pointer_pos {
+                    *current = screen_to_canvas(screen_pos, canvas_center, offset, zoom);
+                }
+            }
             DragState::Resizing {
                 handle,
                 element_id,
@@ -672,25 +701,46 @@ fn handle_select_tool(
                     }
                 }
             }
-            _ => {
+            DragState::None => {
                 session.canvas_state.pan(delta_screen);
             }
+            _ => {}
         }
     }
 
-    if response.drag_stopped() {
+    if response.drag_stopped_by(egui::PointerButton::Primary) {
         match &session.drag_state {
             DragState::Moving { .. } => session.record_edit("Move Element", ChangeSource::Human),
             DragState::Resizing { .. } => {
                 session.record_edit("Resize Element", ChangeSource::Human)
             }
+            DragState::MarqueeSelecting { start, current } => {
+                let marquee = egui::Rect::from_two_pos(*start, *current);
+                let marquee_hits: Vec<String> = session
+                    .drawing_elements
+                    .iter()
+                    .filter(|e| e.bounding_rect().intersects(marquee))
+                    .map(|e| e.id.clone())
+                    .collect();
+
+                if response.ctx.input(|i| i.modifiers.shift) {
+                    for id in marquee_hits {
+                        if !session.selected_element_ids.contains(&id) {
+                            session.selected_element_ids.push(id);
+                        }
+                    }
+                } else {
+                    session.selected_element_ids = marquee_hits;
+                }
+            }
             _ => {}
         }
         session.drag_state = DragState::None;
     }
 
-    if response.clicked() && !response.dragged() {
+    if response.clicked_by(egui::PointerButton::Primary) && !response.dragged() {
         if let Some(screen_pos) = pointer_pos {
+            let shift_held = response.ctx.input(|i| i.modifiers.shift);
             let canvas_pos = screen_to_canvas(screen_pos, canvas_center, offset, zoom);
             let hit = session
                 .drawing_elements
@@ -698,7 +748,19 @@ fn handle_select_tool(
                 .rev()
                 .find(|e| e.contains_point(canvas_pos));
 
-            session.selected_element_id = hit.map(|e| e.id.clone());
+            if shift_held {
+                if let Some(el) = hit {
+                    if session.selected_element_ids.contains(&el.id) {
+                        session.selected_element_ids.retain(|id| id != &el.id);
+                    } else {
+                        session.selected_element_ids.push(el.id.clone());
+                    }
+                }
+            } else if let Some(el) = hit {
+                session.selected_element_ids = vec![el.id.clone()];
+            } else {
+                session.selected_element_ids.clear();
+            }
         }
     }
 }
@@ -711,7 +773,7 @@ fn handle_shape_tool(
     zoom: f32,
     pointer_pos: Option<egui::Pos2>,
 ) {
-    if response.drag_started() {
+    if response.drag_started_by(egui::PointerButton::Primary) {
         if let Some(screen_pos) = pointer_pos {
             let canvas_pos = screen_to_canvas(screen_pos, canvas_center, offset, zoom);
             session.drag_state = DragState::Creating {
@@ -721,7 +783,7 @@ fn handle_shape_tool(
         }
     }
 
-    if response.dragged() {
+    if response.dragged_by(egui::PointerButton::Primary) {
         if let DragState::Creating { current, .. } = &mut session.drag_state {
             if let Some(screen_pos) = pointer_pos {
                 *current = screen_to_canvas(screen_pos, canvas_center, offset, zoom);
@@ -729,7 +791,7 @@ fn handle_shape_tool(
         }
     }
 
-    if response.drag_stopped() {
+    if response.drag_stopped_by(egui::PointerButton::Primary) {
         if let DragState::Creating { start, current } = &session.drag_state {
             let start = *start;
             let current = *current;
@@ -768,7 +830,7 @@ fn handle_shape_tool(
                 };
 
                 let element = DrawingElement::new(shape, ShapeStyle::default());
-                session.selected_element_id = Some(element.id.clone());
+                session.selected_element_ids = vec![element.id.clone()];
                 session.drawing_elements.push(element);
                 session.record_edit(&format!("Draw {}", tool_label), ChangeSource::Human);
             }
@@ -868,7 +930,11 @@ impl eframe::App for AgCanvasApp {
         if toggle_history {
             self.show_history_panel = !self.show_history_panel;
         }
-        if delete_selected && !self.show_text_input && !self.show_mermaid_dialog {
+        if delete_selected
+            && !self.show_text_input
+            && !self.show_mermaid_dialog
+            && !self.active_session().selected_element_ids.is_empty()
+        {
             self.active_session_mut().delete_selected();
             self.active_session_mut()
                 .record_edit("Delete Element", ChangeSource::Human);
@@ -1335,7 +1401,7 @@ impl eframe::App for AgCanvasApp {
                     self.active_session_mut().drawing_elements.push(element);
                     self.active_session_mut()
                         .record_edit("Draw Text", ChangeSource::Human);
-                    self.active_session_mut().selected_element_id = Some(eid);
+                    self.active_session_mut().selected_element_ids = vec![eid];
                 }
                 self.show_text_input = false;
                 self.text_input_buffer.clear();
@@ -1353,17 +1419,9 @@ impl eframe::App for AgCanvasApp {
             let response = CanvasInteraction::allocate_canvas(ui);
             let canvas_center = response.rect.center();
 
-            let active_tool = self.active_session().active_tool;
+            if response.dragged_by(egui::PointerButton::Middle) {
-            let is_select_idle = active_tool == Tool::Select
+                self.active_session_mut().canvas_state.pan(response.drag_delta());
-                && matches!(self.active_session().drag_state, DragState::None);
+            }
-
-            if is_select_idle {
-                CanvasInteraction::handle(
-                    ui,
-                    &response,
-                    &mut self.active_session_mut().canvas_state,
-                );
-            } else if active_tool != Tool::Select {
             if response.hovered() {
                 let scroll_delta = ui.input(|i| i.smooth_scroll_delta.y);
                 if scroll_delta != 0.0 {
@@ -1377,11 +1435,13 @@ impl eframe::App for AgCanvasApp {
                     }
                 }
             }
-                if response.dragged_by(egui::PointerButton::Middle) {
+
-                    self.active_session_mut()
+            let is_idle = matches!(self.active_session().drag_state, DragState::None);
-                        .canvas_state
+            if is_idle
-                        .pan(response.drag_delta());
+                && response.dragged_by(egui::PointerButton::Primary)
-                }
+                && ui.input(|i| i.modifiers.command)
+            {
+                self.active_session_mut().canvas_state.pan(response.drag_delta());
             }
 
             if !self.show_mermaid_dialog && !self.show_text_input {
@@ -1424,10 +1484,15 @@ impl eframe::App for AgCanvasApp {
                 zoom,
             );
 
-            if let Some(selected_el) = self.active_session().selected_element() {
+            for selected_el in self.active_session().selected_elements() {
                 draw_selection(&painter, selected_el, canvas_center, offset, zoom);
             }
 
+            if let DragState::MarqueeSelecting { start, current } = &self.active_session().drag_state
+            {
+                draw_marquee(&painter, *start, *current, canvas_center, offset, zoom);
+            }
+
             let drag_state = self.active_session().drag_state.clone();
             draw_creation_preview(
                 &painter,

crates/agcanvas/src/canvas/interaction.rs

@@ -1,34 +1,8 @@
-use super::CanvasState;
 use egui::{Response, Sense, Ui};
 
 pub struct CanvasInteraction;
 
 impl CanvasInteraction {
-    pub fn handle(ui: &Ui, response: &Response, state: &mut CanvasState) {
-        if response.dragged_by(egui::PointerButton::Middle)
-            || (response.dragged_by(egui::PointerButton::Primary)
-                && ui.input(|i| i.modifiers.command))
-        {
-            state.pan(response.drag_delta());
-        }
-
-        if response.hovered() {
-            let scroll_delta = ui.input(|i| i.smooth_scroll_delta.y);
-            if scroll_delta != 0.0 {
-                let zoom_factor = 1.0 + scroll_delta * 0.001;
-                if let Some(pointer_pos) = response.hover_pos() {
-                    state.zoom_at(pointer_pos, response.rect.center(), zoom_factor);
-                }
-            }
-
-            ui.input(|i| {
-                if i.key_pressed(egui::Key::Num0) && i.modifiers.command {
-                    state.reset();
-                }
-            });
-        }
-    }
-
     pub fn allocate_canvas(ui: &mut Ui) -> Response {
         let available_size = ui.available_size();
         let (rect, response) = ui.allocate_exact_size(available_size, Sense::click_and_drag());

crates/agcanvas/src/drawing/mod.rs

@@ -6,7 +6,7 @@ mod tool;
 pub use boolean::BooleanOpType;
 pub use element::{DrawingElement, Shape, ShapeStyle};
 pub use render::{
-    draw_creation_preview, draw_elements, draw_selection, find_handle_at_screen_pos,
+    draw_creation_preview, draw_elements, draw_marquee, draw_selection, find_handle_at_screen_pos,
     screen_to_canvas,
 };
 pub use tool::{DragState, Tool};

crates/agcanvas/src/drawing/render.rs

@@ -134,6 +134,80 @@ pub fn draw_selection(
     }
 }
 
+pub fn draw_marquee(
+    painter: &Painter,
+    start: Pos2,
+    current: Pos2,
+    canvas_center: Pos2,
+    offset: Vec2,
+    zoom: f32,
+) {
+    let screen_start = canvas_to_screen(start, canvas_center, offset, zoom);
+    let screen_current = canvas_to_screen(current, canvas_center, offset, zoom);
+    let rect = egui::Rect::from_two_pos(screen_start, screen_current);
+    let fill = Color32::from_rgba_premultiplied(
+        SELECTION_COLOR.r(),
+        SELECTION_COLOR.g(),
+        SELECTION_COLOR.b(),
+        36,
+    );
+
+    painter.rect_filled(rect, 0.0, fill);
+    draw_dashed_rect(painter, rect, Stroke::new(1.5, SELECTION_COLOR), 8.0, 4.0);
+}
+
+fn draw_dashed_rect(painter: &Painter, rect: egui::Rect, stroke: Stroke, dash: f32, gap: f32) {
+    draw_dashed_line(
+        painter,
+        rect.left_top(),
+        rect.right_top(),
+        stroke,
+        dash,
+        gap,
+    );
+    draw_dashed_line(
+        painter,
+        rect.right_top(),
+        rect.right_bottom(),
+        stroke,
+        dash,
+        gap,
+    );
+    draw_dashed_line(
+        painter,
+        rect.right_bottom(),
+        rect.left_bottom(),
+        stroke,
+        dash,
+        gap,
+    );
+    draw_dashed_line(
+        painter,
+        rect.left_bottom(),
+        rect.left_top(),
+        stroke,
+        dash,
+        gap,
+    );
+}
+
+fn draw_dashed_line(painter: &Painter, from: Pos2, to: Pos2, stroke: Stroke, dash: f32, gap: f32) {
+    let dir = to - from;
+    let len = dir.length();
+    if len <= 0.0 {
+        return;
+    }
+
+    let dir_norm = dir / len;
+    let mut covered = 0.0;
+    while covered < len {
+        let seg_start = from + dir_norm * covered;
+        let seg_end = from + dir_norm * (covered + dash).min(len);
+        painter.line_segment([seg_start, seg_end], stroke);
+        covered += dash + gap;
+    }
+}
+
 fn draw_path_fill(
     painter: &Painter,
     polygon: &PathPolygon,

crates/agcanvas/src/drawing/tool.rs

@@ -45,7 +45,11 @@ pub enum DragState {
         current: Pos2,
     },
     Moving {
-        element_id: String,
+        element_ids: Vec<String>,
+    },
+    MarqueeSelecting {
+        start: Pos2,
+        current: Pos2,
     },
     Resizing {
         handle: ResizeHandle,

crates/agcanvas/src/session.rs

@@ -88,7 +88,7 @@ pub struct Session {
     pub description_text: String,
 
     pub drawing_elements: Vec<DrawingElement>,
-    pub selected_element_id: Option<String>,
+    pub selected_element_ids: Vec<String>,
     pub active_tool: Tool,
     pub drag_state: DragState,
     pub history: HistoryTree,
@@ -110,7 +110,7 @@ impl Session {
             svg_source: None,
             description_text: String::new(),
             drawing_elements: Vec::new(),
-            selected_element_id: None,
+            selected_element_ids: Vec::new(),
             active_tool: Tool::default(),
             drag_state: DragState::default(),
             history: HistoryTree::new(DocumentSnapshot::new_empty()),
@@ -145,26 +145,37 @@ impl Session {
         self.svg_source = None;
         self.description_text.clear();
         self.drawing_elements.clear();
-        self.selected_element_id = None;
+        self.selected_element_ids.clear();
         self.drag_state = DragState::default();
         self.canvas_state.reset();
     }
 
+    #[allow(dead_code)]
     pub fn selected_element(&self) -> Option<&DrawingElement> {
-        self.selected_element_id
+        self.selected_element_ids
-            .as_ref()
+            .iter()
-            .and_then(|id| self.drawing_elements.iter().find(|e| e.id == *id))
+            .find_map(|id| self.drawing_elements.iter().find(|e| e.id == *id))
+    }
+
+    pub fn selected_elements(&self) -> Vec<&DrawingElement> {
+        self.selected_element_ids
+            .iter()
+            .filter_map(|id| self.drawing_elements.iter().find(|e| e.id == *id))
+            .collect()
     }
 
     #[allow(dead_code)]
     pub fn selected_element_mut(&mut self) -> Option<&mut DrawingElement> {
-        let id = self.selected_element_id.clone();
+        let id = self.selected_element_ids.first().cloned();
         id.and_then(move |id| self.drawing_elements.iter_mut().find(|e| e.id == id))
     }
 
     pub fn delete_selected(&mut self) {
-        if let Some(id) = self.selected_element_id.take() {
+        if !self.selected_element_ids.is_empty() {
-            self.drawing_elements.retain(|e| e.id != id);
+            let selected_ids = self.selected_element_ids.clone();
+            self.drawing_elements
+                .retain(|e| !selected_ids.contains(&e.id));
+            self.selected_element_ids.clear();
         }
     }
 
@@ -182,7 +193,7 @@ impl Session {
         self.svg_renderer = None;
         self.svg_texture = None;
         self.description_text.clear();
-        self.selected_element_id = None;
+        self.selected_element_ids.clear();
         self.drag_state = DragState::default();
     }
 
@@ -194,7 +205,7 @@ impl Session {
             self.svg_renderer = None;
             self.svg_texture = None;
             self.description_text.clear();
-            self.selected_element_id = None;
+            self.selected_element_ids.clear();
             self.drag_state = DragState::default();
             true
         } else {
@@ -210,7 +221,7 @@ impl Session {
             self.svg_renderer = None;
             self.svg_texture = None;
             self.description_text.clear();
-            self.selected_element_id = None;
+            self.selected_element_ids.clear();
             self.drag_state = DragState::default();
             true
         } else {