Highlight the drag destination's current drop target area during drag & drop to the composer
One of the things that I've struggled with for some years in Evolution, and I'm realizing only now, is the lack of very clear visual indication of where your drag and drop is being targetted in the composer, as the composer distinguishes (or used to distinguish) between attaching a file for the attachment bar vs "inserting" an image into the HTML body.
I see there are two methods available for this: https://developer.gnome.org/gtk3/stable/gtk3-Drag-and-Drop.html#gtk-drag-highlight and https://developer.gnome.org/gtk3/stable/gtk3-Drag-and-Drop.html#gtk-drag-unhighlight
I think it would be beneficial to tell GTK to highlight/unhighlight the relevant widgets depending on what it's currently interpreting, be it the composer's webkit view or the attachment bar expander/pane widget, so that I can be sure where my drag is going. It happened to me many times in the past that I wanted to add an attachment and it ended up inserting the file (or worse, the URI) directly into the mail's text body.