live-preview: Changing number type shouldn't create a property if one has not already been set.

[NigelBreslaw]

Bug Description

As per #5929 you can scroll the list of visual properties with a touchpad or mouse wheel. If the cursor happens to pass over some combo boxes there values will change. This causes two issues:

1. Unexpected change to value types e.g. px -> rem. Windows, Mac and Linux don't support scrolling comboboxes by default.
2. If there was no value e.g. x was set to nothing. Changing from px -> rem now adds the property to the code e.g. x: 0rem causing the whole UI to change.

If I had not been recording a screencast I don't know how I would have even realised what had happened. There was enough lag between the changes and the live-preview updating it wasn't even clear what had happened. Just the app suddenly looked wrong and some new code had magically shown up. Even if the issue linked to at the top is fixed we probably should not be adding in code when there is no property but you change the type? @szecket

Reproducible Code (if applicable)

Environment Details

• Slint Version: n/a
• Platform/OS: n/a
• Programming Language: n/a
• Backend/Renderer: n/a
• Slint extension live-preview: production and nightly.

Product Impact

No response

[szecket]

Agreed that this is ungood. At the very least making the value not change if the unit changes is a good start. Using the scrollbar is an inelegant solution.
Is it possible to only use scroll or the select is open? That would be the best solution