I DO have trouble opening any .h that does not appear in the tabs across the top tab bar. Both the .c & .h are present in the sketch’s directory but are listed as simply two identical entries in the ‘Tab Menu’ as & . Clicking on either of them only pulls up <file.c>.
I have deleted both entries via the "Tab Menu’=>‘Delete’ (the second of which does pull up the .h). Then I manually move copies of .<c,h> back into the sketch directory, and open the .h from the SDE sketch. This results in a new duplicate Arduino SDE window with 2 identical entries in the ‘Tab Menu’ neither of which will pull up the <file.h>.
Note: The visible tabs across top of the tab bar are properly labeled .c and .h. I have the sense that if I could scroll the tab bar to the right, I’d see the .h tab and would consequently be able to access it.
In case it’s relevant, my sketch comprises 13 .<c,h> pairs.