gpui: Fix img aspect ratio overriding existing value

[zaknesler]

Objective

The gpui::img element always overrides the aspect_ratio field, so if you have an image element that applies its own .aspect_ratio() it just gets wiped out.

Solution

Only apply the aspect ratio default if one is not already set.

Testing

It's a very minor change but I did add a small test to ensure it actually gets overridden.

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content adheres to Zed's UI standards (UX/UI and icon guidelines)
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Showcase

This came from an issue where vertical images inside an element (square in this case, as you'd see in an image gallery) do not behave correctly with object_fit values of ObjectFit::Contain or ObjectFit::ScaleDown.

This was simply because despite the image element having a fixed square size (img().size(px(200.))), the aspect ratio would be forced to the ratio of the image itself, so vertical images weren't being properly fitted into their containers.

Minimal repro for that issue: https://github.com/zaknesler/gpui-object-fit

So with this change, you can set .aspect_square() and the object fit will behave as you'd expect:

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Release Notes:

• gpui: Fix image element's aspect ratio overriding existing value

[MrSubidubi]

Nice catch, thank you! Also appreciate you adding a test upfront for this!