Core behavior and first-principles view

Layouts are the geometry engine of Qt interfaces. Strong layout design removes fragile pixel-level positioning and makes UI behavior predictable across window sizes.

Layout hierarchy should map to semantic groups, not just visual convenience.

Size policies and stretch factors drive how space is redistributed during resize.

Margins and spacing are functional readability tools, not cosmetic leftovers.

Low-level model and equations

Simplified stretch-based width allocation:

Where:

• $w_i$: width of item $i$
• $s_i$: stretch factor of item $i$

How to build this correctly in practice

1. Replace fixed geometry with top-level and nested layout managers.
2. Tune stretch and policy values while testing minimum and maximum window sizes.
3. Group related controls in sub-layouts for maintainable structure.
4. Capture layout snapshots during QA for regression comparison.

Common failure patterns and review checks

• Mixing fixed geometry and managed layout creates unstable behavior.
• Unreviewed size policy defaults can cause clipping or wasted space.
• Deep unstructured nesting increases maintenance cost.
• Layout tests should include localization and long-string scenarios.

Good layout architecture lets UI scale in size and content without repeated manual coordinate fixes.