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_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
-size 35171
+oid sha256:d27392f625445e82285c8eaeb532e11dcb02030f24c7a769356cb4c23d7c2067
+size 41516