Window: move only by dragging title bar (#8183)

* Part of https://github.com/emilk/egui/issues/8180

So far, you've been able to move any `egui::Window` by dragging anywhere
on it. This makes sense on touch screens with thick fingers, but less so
on non-touch-screens.

With this PR, you can now control it with a new enum `WindowDrag`

crates/egui/src/containers/area.rs

@@ -583,7 +583,7 @@ impl Area {
     }
 }
 
-fn round_area_position(ctx: &Context, pos: Pos2) -> Pos2 {
+pub(crate) fn round_area_position(ctx: &Context, pos: Pos2) -> Pos2 {
     // We round a lot of rendering to pixels, so we round the whole
     // area positions to pixels too, so avoid widgets appearing to float
     // around independently of each other when the area is dragged.

crates/egui/src/containers/mod.rs

@@ -32,5 +32,5 @@ pub use {
     scroll_area::ScrollArea,
     sides::Sides,
     tooltip::*,
-    window::Window,
+    window::{Window, WindowDrag},
 };

crates/egui/src/containers/window.rs

@@ -9,6 +9,54 @@ use crate::*;
 use super::scroll_area::{DragScroll, ScrollBarVisibility, ScrollSource};
 use super::{Area, Frame, Resize, ScrollArea, area, resize};
 
+/// Where the user can drag to move a [`Window`].
+///
+/// See [`Window::drag_area`].
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Default)]
+#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
+pub enum WindowDrag {
+    /// Window cannot be moved by dragging.
+    ///
+    /// [`Window::movable(false)`](Window::movable) forces this regardless of
+    /// what was passed to [`Window::drag_area`].
+    Off,
+
+    /// The user can drag the window from anywhere on its surface.
+    ///
+    /// Good for touch screens, but can interfere with selecting / dragging
+    /// content inside the window when used with a mouse.
+    Anywhere,
+
+    /// Only the title bar accepts the move-drag gesture.
+    ///
+    /// Windows without a title bar (see [`Window::title_bar`]) silently fall
+    /// back to [`Self::Anywhere`] — otherwise they'd be unmovable.
+    TitleBar,
+
+    /// [`Self::Anywhere`] when a touch screen is detected (see
+    /// [`crate::InputState::has_touch_screen`]); [`Self::TitleBar`] otherwise.
+    /// The recommended default.
+    #[default]
+    OnTouch,
+}
+
+impl WindowDrag {
+    /// Resolve [`Self::OnTouch`] to either [`Self::Anywhere`] or [`Self::TitleBar`]
+    /// based on whether a touch screen was detected.
+    fn resolve(self, ctx: &Context) -> Self {
+        match self {
+            Self::OnTouch => {
+                if ctx.input(|i| i.has_touch_screen()) {
+                    Self::Anywhere
+                } else {
+                    Self::TitleBar
+                }
+            }
+            other => other,
+        }
+    }
+}
+
 /// Builder for a floating window which can be dragged, closed, collapsed, resized and scrolled (off by default).
 ///
 /// You can customize:
@@ -43,6 +91,7 @@ pub struct Window<'a> {
     with_title_bar: bool,
     fade_out: bool,
     auto_sized: bool,
+    drag_area: WindowDrag,
 }
 
 impl<'a> Window<'a> {
@@ -67,6 +116,7 @@ impl<'a> Window<'a> {
             with_title_bar: true,
             fade_out: true,
             auto_sized: false,
+            drag_area: WindowDrag::default(),
         }
     }
 
@@ -142,12 +192,29 @@ impl<'a> Window<'a> {
     }
 
     /// If `false` the window will be immovable.
+    ///
+    /// If `true`, you can move the window by dragging it.
+    /// Where you can drag to move the window is determined by [`Self::drag_area`].
     #[inline]
     pub fn movable(mut self, movable: bool) -> Self {
         self.area = self.area.movable(movable);
         self
     }
 
+    /// Where the user can grab the window to move it.
+    ///
+    /// Defaults to [`WindowDrag::OnTouch`]: drag anywhere on touch screens,
+    /// title bar only otherwise. See [`WindowDrag`] for details.
+    ///
+    /// [`Self::movable(false)`](Self::movable) forces [`WindowDrag::Off`]
+    /// regardless of this setting. Windows without a title bar (see
+    /// [`Self::title_bar`]) fall back to [`WindowDrag::Anywhere`].
+    #[inline]
+    pub fn drag_area(mut self, drag_area: WindowDrag) -> Self {
+        self.drag_area = drag_area;
+        self
+    }
+
     /// `order(Order::Foreground)` for a Window that should always be on top
     #[inline]
     pub fn order(mut self, order: Order) -> Self {
@@ -489,8 +556,64 @@ impl Window<'_> {
             with_title_bar,
             fade_out,
             auto_sized,
+            drag_area: drag_area_setting,
         } = self;
 
+        // `Window::movable(false)` (and `Area::movable(false)`) and
+        // `WindowDrag::Off` both mean "this window cannot be moved by
+        // dragging". Without a title bar, `TitleBar` mode would leave the
+        // window unmovable, so silently fall back to drag-anywhere instead.
+        let effective_drag = if !area.is_movable() || drag_area_setting == WindowDrag::Off {
+            WindowDrag::Off
+        } else if !with_title_bar {
+            WindowDrag::Anywhere
+        } else {
+            drag_area_setting.resolve(ctx)
+        };
+
+        // Make the area itself agree: keep its movable flag in sync with
+        // the resolved drag mode so resize behavior and `Area::begin`'s
+        // drag-from-anywhere handling don't disagree with the title-bar
+        // path. (Builder order shouldn't matter — `.drag_area(Off)` after
+        // `.movable(true)` and vice versa both end up here.)
+        let area = if effective_drag == WindowDrag::Off {
+            area.movable(false)
+        } else {
+            area
+        };
+
+        // Apply the previous frame's title-bar drag _before_ `Area::begin`
+        // loads the state. We can't apply it inside the content closure because
+        // `Area::end` writes the locally-captured `AreaState` back, overwriting
+        // any in-frame mutation.
+        //
+        // We deliberately leave `Area` with its normal `Sense::DRAG`: that way
+        // the area's widget still absorbs drag hit-tests over the body, so the
+        // resize-edge widgets aren't picked as the "closest drag" target when
+        // hovering anywhere in the window. The drag-from-anywhere move that
+        // `Area::begin` would then apply is undone right after `begin` for
+        // `WindowDrag::TitleBar`.
+        let title_drag_mode = effective_drag == WindowDrag::TitleBar;
+        let pivot_pos_before_begin = if title_drag_mode {
+            if let Some(resp) = ctx.read_response(area.id.with("__title_click"))
+                && resp.dragged()
+            {
+                let delta = ctx.input(|i| i.pointer.delta());
+                if delta != Vec2::ZERO {
+                    ctx.memory_mut(|mem| {
+                        if let Some(state) = mem.areas_mut().get_mut(area.id)
+                            && let Some(pivot_pos) = state.pivot_pos.as_mut()
+                        {
+                            *pivot_pos += delta;
+                        }
+                    });
+                }
+            }
+            area::AreaState::load(ctx, area.id).and_then(|s| s.pivot_pos)
+        } else {
+            None
+        };
+
         let style = ctx.global_style();
 
         let window_frame = frame.unwrap_or_else(|| Frame::window(&style));
@@ -525,6 +648,24 @@ impl Window<'_> {
         let on_top = Some(area_layer_id) == ctx.top_layer_id();
         let mut area = area.begin(ctx);
 
+        // Title-bar-drag mode: throw away any drag-from-anywhere movement
+        // `Area::begin` may have applied. The title-bar pre-begin step above
+        // already accounted for the title drag. We then re-run the same
+        // constrain+round step `Area::begin` does so the title-bar drag
+        // can't escape `constrain_rect` or reintroduce sub-pixel jitter.
+        if let Some(pre_begin_pivot) = pivot_pos_before_begin {
+            let constrain = area.constrain();
+            let constrain_rect = area.constrain_rect();
+            let state = area.state_mut();
+            state.pivot_pos = Some(pre_begin_pivot);
+            if constrain {
+                state.set_left_top_pos(
+                    Context::constrain_window_rect_to_area(state.rect(), constrain_rect).min,
+                );
+            }
+            state.set_left_top_pos(area::round_area_position(ctx, state.left_top_pos()));
+        }
+
         area.with_widget_info(|| {
             WidgetInfo::labeled(
                 WidgetType::Window,
@@ -576,6 +717,8 @@ impl Window<'_> {
                             on_top,
                             open.as_deref_mut(),
                             auto_sized,
+                            effective_drag == WindowDrag::TitleBar,
+                            area_id,
                         );
                     }
                     collapsing
@@ -1142,6 +1285,8 @@ fn title_ui(
     active: bool,
     open: Option<&mut bool>,
     auto_sized: bool,
+    drag_to_move: bool,
+    area_id: Id,
 ) -> Response {
     let shape_idx = ui.painter().add(Shape::Noop);
 
@@ -1247,17 +1392,22 @@ fn title_ui(
         }
     }
 
-    if collapsible
+    if collapsible || drag_to_move {
-        && child_ui
+        // Single widget covers double-click-to-toggle (when collapsible) and
-            .interact(
+        // drag-to-move (in title-bar-drag mode). The move itself is applied in
-                title_click_rect,
+        // `Window::show_dyn` _before_ `Area::begin` next frame, since
-                child_ui.auto_id_with("window_title_click"),
+        // `Area::end` overwrites any in-frame mutation of `AreaState`.
-                Sense::click(),
+        let sense = if drag_to_move {
-            )
+            Sense::click_and_drag()
-            .double_clicked()
+        } else {
-    {
+            Sense::click()
+        };
+        let response = child_ui.interact(title_click_rect, area_id.with("__title_click"), sense);
+
+        if collapsible && response.double_clicked() {
             collapsing.toggle(&child_ui);
         }
+    }
 
     {
         let mut header_frame = frame.shadow(Shadow::NONE);

crates/egui/src/memory/mod.rs

@@ -1171,6 +1171,10 @@ impl Areas {
         self.areas.get(&id)
     }
 
+    pub(crate) fn get_mut(&mut self, id: Id) -> Option<&mut area::AreaState> {
+        self.areas.get_mut(&id)
+    }
+
     /// All layers back-to-front, top is last.
     pub(crate) fn order(&self) -> &[LayerId] {
         &self.order

crates/egui_demo_lib/src/demo/window_options.rs

@@ -1,4 +1,4 @@
-use egui::{UiKind, Vec2b};
+use egui::{UiKind, Vec2b, WindowDrag};
 
 #[derive(Clone, PartialEq)]
 #[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
@@ -7,6 +7,7 @@ pub struct WindowOptions {
     title_bar: bool,
     closable: bool,
     collapsible: bool,
+    movable: bool,
     resizable: bool,
     constrain: bool,
     scroll2: Vec2b,
@@ -15,6 +16,8 @@ pub struct WindowOptions {
     anchored: bool,
     anchor: egui::Align2,
     anchor_offset: egui::Vec2,
+
+    drag_area: WindowDrag,
 }
 
 impl Default for WindowOptions {
@@ -24,6 +27,7 @@ impl Default for WindowOptions {
             title_bar: true,
             closable: true,
             collapsible: true,
+            movable: true,
             resizable: true,
             constrain: true,
             scroll2: Vec2b::TRUE,
@@ -31,6 +35,7 @@ impl Default for WindowOptions {
             anchored: false,
             anchor: egui::Align2::RIGHT_TOP,
             anchor_offset: egui::Vec2::ZERO,
+            drag_area: WindowDrag::default(),
         }
     }
 }
@@ -46,6 +51,7 @@ impl crate::Demo for WindowOptions {
             title_bar,
             closable,
             collapsible,
+            movable,
             resizable,
             constrain,
             scroll2,
@@ -53,6 +59,7 @@ impl crate::Demo for WindowOptions {
             anchored,
             anchor,
             anchor_offset,
+            drag_area,
         } = self.clone();
 
         let enabled = ui.input(|i| i.time) - disabled_time > 2.0;
@@ -66,7 +73,9 @@ impl crate::Demo for WindowOptions {
             .resizable(resizable)
             .constrain(constrain)
             .collapsible(collapsible)
+            .movable(movable)
             .title_bar(title_bar)
+            .drag_area(drag_area)
             .scroll(scroll2)
             .constrain_to(ui.available_rect_before_wrap())
             .enabled(enabled);
@@ -87,6 +96,7 @@ impl crate::View for WindowOptions {
             title_bar,
             closable,
             collapsible,
+            movable,
             resizable,
             constrain,
             scroll2,
@@ -94,6 +104,7 @@ impl crate::View for WindowOptions {
             anchored,
             anchor,
             anchor_offset,
+            drag_area,
         } = self;
         ui.horizontal(|ui| {
             ui.label("title:");
@@ -106,6 +117,8 @@ impl crate::View for WindowOptions {
                     ui.checkbox(title_bar, "title_bar");
                     ui.checkbox(closable, "closable");
                     ui.checkbox(collapsible, "collapsible");
+                    ui.checkbox(movable, "movable")
+                        .on_hover_text("Can the window be moved by dragging?");
                     ui.checkbox(resizable, "resizable");
                     ui.checkbox(constrain, "constrain")
                         .on_hover_text("Constrain window to the screen");
@@ -140,6 +153,19 @@ impl crate::View for WindowOptions {
             });
         });
 
+        ui.horizontal(|ui| {
+            ui.label("Drag to move:")
+                .on_hover_text("Where the user can grab the window to move it");
+            ui.selectable_value(drag_area, WindowDrag::Off, "Off")
+                .on_hover_text("The window cannot be dragged to move it (same as movable = false)");
+            ui.selectable_value(drag_area, WindowDrag::OnTouch, "OnTouch")
+                .on_hover_text("Anywhere on touch screens, title-bar only otherwise (default)");
+            ui.selectable_value(drag_area, WindowDrag::TitleBar, "TitleBar")
+                .on_hover_text("Only the title bar moves the window");
+            ui.selectable_value(drag_area, WindowDrag::Anywhere, "Anywhere")
+                .on_hover_text("Drag anywhere on the window to move it");
+        });
+
         ui.separator();
         let on_top = Some(ui.layer_id()) == ui.ctx().top_layer_id();
         ui.label(format!("This window is on top: {on_top}."));

crates/egui_demo_lib/tests/snapshots/demos/Window Options.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:ab0a8201038b2b066c5aff1fd1a35bc7aed95e74cf50c293eee4a76d66623822
+oid sha256:d27392f625445e82285c8eaeb532e11dcb02030f24c7a769356cb4c23d7c2067
-size 35171
+size 41516

tests/egui_tests/tests/test_window_drag.rs

@@ -0,0 +1,195 @@
+//! Tests for [`Window::drag_area`] and [`Window::movable`].
+//!
+//! Each test sets up a window with a particular drag configuration, drags
+//! either inside or outside the title bar, and asserts on the area rect's
+//! delta. `WindowDrag::OnTouch` is not exercised here since it just resolves
+//! to `TitleBar` (no touch screen in headless tests).
+
+use egui::{Id, Pos2, Sense, Vec2, Window, WindowDrag};
+use egui_kittest::Harness;
+
+struct State {
+    drag_area: WindowDrag,
+    movable: bool,
+}
+
+fn build(state: State) -> Harness<'static, State> {
+    let mut harness = Harness::builder()
+        .with_size(Vec2::new(500.0, 400.0))
+        .with_max_steps(40) // Area requests a repaint every frame while pressed.
+        .build_ui_state(
+            move |ui, state: &mut State| {
+                Window::new("test_win")
+                    .id(Id::new("test_win"))
+                    .drag_area(state.drag_area)
+                    .movable(state.movable)
+                    .default_pos([100.0, 80.0])
+                    .default_size([180.0, 140.0])
+                    .show(ui.ctx(), |ui| {
+                        // A passive widget fills the body; it has no drag sense
+                        // of its own, so the Area / title-bar widget is what
+                        // decides whether a drag moves the window.
+                        ui.allocate_response(ui.available_size(), Sense::hover());
+                    });
+            },
+            state,
+        );
+    // Let the window settle (auto-position / size, then idle).
+    harness.run_steps(4);
+    harness
+}
+
+fn window_rect(harness: &Harness<'_, State>) -> egui::Rect {
+    egui::AreaState::load(&harness.ctx, Id::new("test_win"))
+        .expect("window area should be persisted after the first frame")
+        .rect()
+}
+
+/// Drag the pointer from `from` to `to` over multiple frames; release at the end.
+fn drag(harness: &mut Harness<'_, State>, from: Pos2, to: Pos2) {
+    harness.drag_at(from);
+    harness.run_steps(4);
+    harness.hover_at(to);
+    harness.run_steps(4);
+    harness.drop_at(to);
+    harness.run_steps(4);
+}
+
+fn titlebar_pos(rect: egui::Rect) -> Pos2 {
+    // Just inside the title bar:
+    Pos2::new(rect.center().x, rect.top() + 8.0)
+}
+
+fn body_pos(rect: egui::Rect) -> Pos2 {
+    // Well below the title bar:
+    Pos2::new(rect.center().x, rect.bottom() - 30.0)
+}
+
+#[test]
+fn title_bar_drag_on_titlebar_moves_window() {
+    let mut harness = build(State {
+        drag_area: WindowDrag::TitleBar,
+        movable: true,
+    });
+
+    let before = window_rect(&harness);
+    let from = titlebar_pos(before);
+    let to = from + Vec2::new(60.0, 40.0);
+
+    drag(&mut harness, from, to);
+
+    let after = window_rect(&harness);
+    let moved = after.min - before.min;
+    assert!(
+        20.0 < moved.x && 20.0 < moved.y,
+        "TitleBar + drag on titlebar should move the window (delta = {moved:?})"
+    );
+}
+
+#[test]
+fn title_bar_drag_outside_titlebar_keeps_window_put() {
+    let mut harness = build(State {
+        drag_area: WindowDrag::TitleBar,
+        movable: true,
+    });
+
+    let before = window_rect(&harness);
+    let from = body_pos(before);
+    let to = from + Vec2::new(60.0, -40.0);
+
+    drag(&mut harness, from, to);
+
+    let after = window_rect(&harness);
+    let moved = after.min - before.min;
+    assert!(
+        moved.length() < 1.0,
+        "TitleBar + drag in the body should NOT move the window (delta = {moved:?})"
+    );
+}
+
+#[test]
+fn anywhere_drag_in_body_moves_window() {
+    let mut harness = build(State {
+        drag_area: WindowDrag::Anywhere,
+        movable: true,
+    });
+
+    let before = window_rect(&harness);
+    let from = body_pos(before);
+    let to = from + Vec2::new(60.0, -40.0);
+
+    drag(&mut harness, from, to);
+
+    let after = window_rect(&harness);
+    let moved = after.min - before.min;
+    assert!(
+        20.0 < moved.x && moved.y < -20.0,
+        "Anywhere + drag anywhere should move the window (delta = {moved:?})"
+    );
+}
+
+#[test]
+fn movable_false_keeps_window_put_even_on_titlebar() {
+    // Regression: a `movable(false)` window used to still move when the user
+    // dragged the title bar in `TitleBar` mode.
+    let mut harness = build(State {
+        drag_area: WindowDrag::TitleBar,
+        movable: false,
+    });
+
+    let before = window_rect(&harness);
+    let from = titlebar_pos(before);
+    let to = from + Vec2::new(60.0, 40.0);
+
+    drag(&mut harness, from, to);
+
+    let after = window_rect(&harness);
+    let moved = after.min - before.min;
+    assert!(
+        moved.length() < 1.0,
+        "TitleBar + movable(false) should NOT move the window (delta = {moved:?})"
+    );
+}
+
+#[test]
+fn off_keeps_window_put_on_body_drag() {
+    // `WindowDrag::Off` should freeze the window regardless of `movable`.
+    let mut harness = build(State {
+        drag_area: WindowDrag::Off,
+        movable: true,
+    });
+
+    let before = window_rect(&harness);
+    let from = body_pos(before);
+    let to = from + Vec2::new(60.0, -40.0);
+
+    drag(&mut harness, from, to);
+
+    let after = window_rect(&harness);
+    let moved = after.min - before.min;
+    assert!(
+        moved.length() < 1.0,
+        "Off + drag in the body should NOT move the window (delta = {moved:?})"
+    );
+}
+
+#[test]
+fn off_keeps_window_put_on_titlebar_drag() {
+    let mut harness = build(State {
+        drag_area: WindowDrag::Off,
+        movable: true,
+    });
+
+    let before = window_rect(&harness);
+    let from = titlebar_pos(before);
+    let to = from + Vec2::new(60.0, 40.0);
+
+    drag(&mut harness, from, to);
+
+    let after = window_rect(&harness);
+    let moved = after.min - before.min;
+    assert!(
+        moved.length() < 1.0,
+        "Off + drag on titlebar should NOT move the window (delta = {moved:?})"
+    );
+}