Horizontal layout vertically offsets child UIs

**Describe the bug**

When laying out horizontally a series of allocated vertical layouts, the child uis are offset and un-aligned.

**To Reproduce**
Steps to reproduce the behavior:
1. Clone the reproduction repo https://github.com/clembu/egui_horizontal_offset_bug_reproduction
2. Run the executable: `cargo run`
4. ----> The children are vertically offset

**Expected behavior**
The lists are aligned horizontally, as they are all the same size.

**Screenshots**

**Desktop (please complete the following information):**
- OS: Windows 11
- Browser: N/A
- Version: Windows 24H2 Build 26100.2605

Clone the linked egui_horizontal_offset_bug_reproduction repository and run it with `cargo run` to reproduce the vertical offset shown in the screenshot. Trace the horizontal layout behavior from the reproduction and compare the child positions with the expected aligned lists. Done means the child UIs are vertically aligned when equal-sized vertical layouts are arranged horizontally.

Written by the indexing model from the issue text.