[windows] Constrain windows to screen

CHANGELOG.md

@@ -6,6 +6,7 @@
 * Add ability to override text color with `visuals.override_text_color`
 * Refactored the interface for `egui::app::App`
 * Demo App: Add slider to scale all of Egui
+* Windows are now constrained to the screen
 * Fix a bug where some regions would slowly grow for non-integral scales (`pixels_per_point`).
 
 ## 0.2.0 - 2020-10-10

egui/src/containers/area.rs

@@ -175,8 +175,8 @@ impl Prepared {
 
         state.size = content_ui.min_rect().size();
 
-        let rect = Rect::from_min_size(state.pos, state.size);
-        let clip_rect = Rect::everything(); // TODO: get from context
+        let area_rect = Rect::from_min_size(state.pos, state.size);
+        let clip_rect = ctx.rect();
 
         let interact_id = if movable {
             Some(layer_id.id.with("move"))
@@ -186,7 +186,7 @@ impl Prepared {
         let move_response = ctx.interact(
             layer_id,
             clip_rect,
-            rect,
+            area_rect,
             interact_id,
             Sense::click_and_drag(),
         );
@@ -210,14 +210,7 @@ impl Prepared {
             }
         }
 
-        // Constrain to screen:
-        let margin = 32.0;
-        state.pos = state.pos.max(pos2(margin - state.size.x, 0.0));
-        state.pos = state.pos.min(pos2(
-            ctx.input().screen_size.x - margin,
-            ctx.input().screen_size.y - margin,
-        ));
-        state.pos = ctx.round_pos_to_pixels(state.pos);
+        state.pos = ctx.constrain_window_rect(area_rect).min;
 
         if move_response.active
             || mouse_pressed_on_area(ctx, layer_id)

egui/src/containers/window.rs

@@ -384,11 +384,11 @@ fn interact(
     area_state: &mut area::State,
     resize_id: Id,
 ) -> Option<WindowInteraction> {
-    let new_rect = resize_window(ctx, &window_interaction)?;
-
+    let new_rect = move_and_resize_window(ctx, &window_interaction)?;
     let new_rect = ctx.round_rect_to_pixels(new_rect);
-    // TODO: add this to a Window state instead as a command "move here next frame"
+    let new_rect = ctx.constrain_window_rect(new_rect);
 
+    // TODO: add this to a Window state instead as a command "move here next frame"
     area_state.pos = new_rect.min;
 
     if window_interaction.is_resize() {
@@ -401,7 +401,7 @@ fn interact(
     Some(window_interaction)
 }
 
-fn resize_window(ctx: &Context, window_interaction: &WindowInteraction) -> Option<Rect> {
+fn move_and_resize_window(ctx: &Context, window_interaction: &WindowInteraction) -> Option<Rect> {
     window_interaction.set_cursor(ctx);
     let mouse_pos = ctx.input().mouse.pos?;
     let mut rect = window_interaction.start_rect; // prevent drift

egui/src/context.rs

@@ -165,6 +165,29 @@ impl Context {
 
     // ---------------------------------------------------------------------
 
+    /// Constraint the position of a window/area
+    /// so it fits within the screen.
+    pub(crate) fn constrain_window_rect(&self, window: Rect) -> Rect {
+        let screen = self.rect();
+
+        let mut pos = window.min;
+
+        // Constrain to screen, unless window is too large to fit:
+        let margin_x = (window.width() - screen.width()).at_least(0.0);
+        let margin_y = (window.height() - screen.height()).at_least(0.0);
+
+        pos.x = pos.x.at_least(screen.left() - margin_x);
+        pos.x = pos.x.at_most(screen.right() + margin_x - window.width());
+        pos.y = pos.y.at_least(screen.top() - margin_y);
+        pos.y = pos.y.at_most(screen.bottom() + margin_y - window.height());
+
+        pos = self.round_pos_to_pixels(pos);
+
+        Rect::from_min_size(pos, window.size())
+    }
+
+    // ---------------------------------------------------------------------
+
     /// Call at the start of every frame.
     /// Returns a master fullscreen UI, covering the entire screen.
     pub fn begin_frame(self: &mut Arc<Self>, new_input: RawInput) -> Ui {