[Feature]: Visualization of empty groups

[ichibha]

This issue is derived from #1812 . It is useful if the following function is implemented: displaying something for empty groups to clarify their existence.

[scytacki]

Hi @arnog I'm working with @dennisrcao on this. Is this concord-consortium#2 a good place to add a visualization of empty groups?

[arnog]

Great! Thanks for looking into this. Here’s how I would approach this:

• modify Box.toMarkup() to add a class (ML_empty-group?) if the group is empty and does not contain the caret (if the caret is in an empty group I would just do nothing since at that point the caret makes it clear where you are. The point of making empty groups visible is to draw attention to them when they could otherwise be invisible and difficult to click on)
• Have that class do some styling like setting a minimum width/height, and perhaps a background color. The class should only be active if the mathfield has the focus
• Trigger that behavior off a new configuration option

[scytacki]

To clarify, are you saying we should not identify the empty group in createBox and instead identify it in toMarkup?

It seemed to me that identifying in createBox was better since it is closer to the "model".
Identifying it in toMarkup meant looking for a box that had no siblings, no children, and had an id. The last two parts are easier in createBox because we can just look for a first atom. After identifying this atom, the createBox function could return a box with a property indicating it is a empty group (or a new subclass of Box could be used). So then toMarkup could add the class to it.

[arnog]

You’re right, you still need to identify it as an empty group, and createBox is the best place for it. I would either create a Box subclass, or you might just be able to create a Box with a custom class.