core: Fix constraints for empty layout

- Respect the padding even for empty layouts
 - When there is a non-default alignment, layouts shouldn't have a
   max size: Fix the case for empty layout
 - Remove max constraint in the orthogonal direction of an empty layout

Author: ogoffart

internal/core/layout.rs

@@ -624,10 +624,16 @@ pub fn box_layout_info(
     alignment: LayoutAlignment,
 ) -> LayoutInfo {
     let count = cells.len();
+    let is_stretch = alignment == LayoutAlignment::Stretch;
     if count < 1 {
-        return LayoutInfo { max: 0 as _, ..LayoutInfo::default() };
+        let mut info = LayoutInfo::default();
+        info.min = padding.begin + padding.end;
+        info.preferred = info.min;
+        if is_stretch {
+            info.max = info.min;
+        }
+        return info;
     };
-    let is_stretch = alignment == LayoutAlignment::Stretch;
     let extra_w = padding.begin + padding.end + spacing * (count - 1) as Coord;
     let min = cells.iter().map(|c| c.constraint.min).sum::<Coord>() + extra_w;
     let max = if is_stretch {
@@ -641,12 +647,7 @@ pub fn box_layout_info(
 }
 
 pub fn box_layout_info_ortho(cells: Slice<BoxLayoutCellData>, padding: &Padding) -> LayoutInfo {
-    let count = cells.len();
-    if count < 1 {
-        return LayoutInfo { max: 0 as _, ..LayoutInfo::default() };
-    };
     let extra_w = padding.begin + padding.end;
-
     let mut fold =
         cells.iter().fold(LayoutInfo { stretch: f32::MAX, ..Default::default() }, |a, b| {
             a.merge(&b.constraint)

tests/cases/crashes/layout_deleted_item.slint

@@ -13,7 +13,7 @@ export TestCase := Window {
     property<[int]> model: [1];
     VerticalLayout {
         under := TouchArea {
-            HorizontalLayout {
+            VerticalLayout {
                 for value in model: TouchArea {
                     horizontal-stretch: 5;
                     vertical-stretch: 5;

tests/cases/layout/empty_layout.slint

@@ -0,0 +1,53 @@
+// Copyright © SixtyFPS GmbH <[email protected]>
+// SPDX-License-Identifier: GPL-3.0-only OR LicenseRef-Slint-Royalty-free-2.0 OR LicenseRef-Slint-Software-3.0
+
+export component TestCase inherits Window {
+    width: 300phx;
+    height: 300phx;
+
+    empty1 := Rectangle { HorizontalLayout { } }
+    empty2 := Rectangle { VerticalLayout { spacing: 80px; padding: 2px; } }
+    empty3 := Rectangle { HorizontalLayout { alignment: center; } }
+    empty4 := Rectangle { VerticalLayout { alignment: end; padding-left: 10phx; padding-top: 50phx; } }
+    empty5 := Rectangle {
+        VerticalLayout {
+            alignment: end; padding-left: 10phx; padding-top: 50phx;
+            for _ in 0 : Text { text: "Hello World!"; }
+        }
+    }
+    empty6 := Rectangle {
+        HorizontalLayout {
+            if false: Rectangle { background: red; min-height: 10px; min-width: 10px; }
+            padding-right: 80phx; padding-bottom: 70phx;
+        }
+    }
+
+    out property <bool> t1: empty1.min-height == 0 && empty1.min-width == 0 && empty1.preferred-height == 0 && empty1.preferred-width == 0 && empty1.max-height > 10000px && empty1.max-width == 0;
+    out property <bool> t2: empty2.min-height == 4px && empty2.min-width == 4px && empty2.preferred-height == 4px && empty2.preferred-width == 4px && empty2.max-height == 4px && empty2.max-width > 100000px;
+    out property <bool> t3: empty3.min-height == 0 && empty3.min-width == 0 && empty3.preferred-height == 0 && empty3.preferred-width == 0 && empty3.max-height > 10000px && empty3.max-width > 10000px;
+    out property <bool> t4: empty4.min-height == 50phx && empty4.min-width == 10phx && empty4.preferred-height == 50phx && empty4.preferred-width == 10phx && empty4.max-height > 10000px && empty4.max-width > 10000px;
+    out property <bool> t5: empty5.min-height == 50phx && empty5.min-width == 10phx && empty5.preferred-height == 50phx && empty5.preferred-width == 10phx && empty5.max-height > 10000px && empty5.max-width > 10000px;
+    out property <bool> t6: empty6.min-height == 70phx && empty6.min-width == 80phx && empty6.preferred-height == 70phx && empty6.preferred-width == 80phx && empty6.max-height > 10000px && empty6.max-width == 80phx;
+    out property <bool> test: t1 && t2 && t3 && t4 && t5 && t6;
+}
+
+/*
+
+```cpp
+auto handle = TestCase::create();
+const TestCase &instance = *handle;
+assert(instance.get_test());
+```
+
+
+```rust
+let instance = TestCase::new().unwrap();
+assert!(instance.get_test());
+```
+
+```js
+var instance = new slint.TestCase();
+assert(instance.test);
+```
+
+*/