When you set a context for a custom field and configure it, if you do not choose global for the context, you are presented with a window to select projects.

To select, you have to use the CTRL + Click method. If you do not, and just click, you loose any prior selections and this has caused many a headache, especially of you hit the modify button and do not have a list of the originally selected projects.

I would like to request that the selection methos is changed see below for 3 potential options (option 3 would be my personal preference):

1. a "checkbox" selection method
2. Project selection box - allow you to start typing a project name (or key) and add multiple projects. Interface would be similar to the Component/s field in this issue layout
3. Drag and drop, similar to the interface when you are adding issue types in the Issue Type Schemes interface or the Select from right column and click add/enable to the left in the Enable Structure interface (Structures app).