Is this Raspberry Pi or Arduino IDE?

You need a diagnostic tool from another sketch. you open up that sketch, CTRL-C, close the donor sketch, try to paste it in your current project, and "clipboard has no string"

you have to keep the donor open until you harvest everything you need.

Is this an Arduino IDE feature, or a blessing from Raspbian?

Probably silly question but did you first select the text that you wanted to copy?

It does appear to be Arduino IDE. I can copy from one application on Raspbian, close the application, then paste to another. It's only with the Arduino IDE that I was able reproduce the issue. It also only occurs with multiple instances of the Arduino IDE (created by starting the IDE multiple times, or opening multiple sketches by double clicking them). The issue does not occur with multiple Arduino IDE windows of the same instance (created via File > Open, File > New, File > Sketchbook, or File > Examples.

The issue also occurs with the standard Linux version of the Arduino IDE, so it's not specific to Raspberry Pi.

The issue does not occur on Windows.