Document handling of editor "tabs" / concatenation of multiple sketch files

I have a bug in the Arduino software version 1.8.9. Initially, I wrote my code to have several tabs each with some categorized functions. At first everything compiled well and there were no errors. Programs loaded into my Arduino 2560 and functioned. I made a few tweaks in my code to have some data display on a serial port LCD I had. Things were going well then, suddenly, my code stopped working. I got compile errors for some of my functions not being declared.

In order to get passed these compile errors, I had to add include statements for each tab that contained my functions. I am not sure why this happened. In one of my tabs, I had a function that called a different function under a different tab. I tried adding an #include statement for that tab in that function. I got an error about a "function definition is not allowed here before a {". Things are really weird right now, and I don't know why.

I have an HP8570 with Windows 7 with service pack 2 installed.

No documentation file or test is named. Start by reviewing the reported Arduino 1.8.9 behavior around editor tabs, sketch-file concatenation, and cross-tab function references; document the expected handling and the relevant use of includes, with guidance that resolves the described compile errors.

Written by the indexing model from the issue text.