HorizontalLayout addComponentToIndex uses default slot if index>count

[rolfsmeds]

Description

When adding elements to a HorizontalLayout with addComponentToIndex, if the index provided is greater than the last existing index in the layout, the element always ends up in the left/default slot.

This is clearly caused by the slot being simply ignored if no component exists at the given index:

flow-components/vaadin-ordered-layout-flow-parent/vaadin-ordered-layout-flow/src/main/java/com/vaadin/flow/component/orderedlayout/HorizontalLayout.java

Line 330 in dc62c93

: null;

Expected outcome

Would expect the element to be placed in whatever slot the last (i.e. end-most) existing element is in.

Minimal reproducible example

One would expect this to place Button 4 in the end slot, but it ends up in the left/default slot:

HorizontalLayout hz = new HorizontalLayout();
hz.add(new Button("1"));
hz.addToEnd(new Button("2"));
hz.addToEnd(new Button("3"));
hz.addComponentAtIndex(hz.getComponentCount()), new Button("4"));

Steps to reproduce

1. Add one or more components to the middle or end slot of a HorizontalLayout
2. Add another component using addComponentAtIndex, providing an index >= the current component count
3. Observe the newly added component being in the left/default slot instead of after the last previously added component

Environment

Vaadin version(s): V25.0

Browsers

Issue is not browser related

[rolfsmeds]

For consistency, add(Component c) should probably also add to whatever the end-most existing component's slot is.

This might cause breaking changes in existing apps where add(c) is used together with the slot-specific addToStart, addToMiddle and addToEnd, if add is used after any of the above slot-specific adders.