Highlight all selected respectively Shift+double click searches for the selected string respectively double clicked word always case-sensitive. This is not customizable.
It is possible to use Quick Find by selecting the word, pressing with default key mappings Ctrl+F or Ctrl+Shift+I to start quick find with selected word and hitting key RETURN to highlight all occurrences of selected word with ignoring the case.
Or regular Find is used by selecting the word, pressing with default key mappings twice Ctrl+F or once Alt+F3 to open regular find with selected word, uncheck option Match case if currently checked, check option Highlight all items found if currently not checked and hitting key RETURN to highlight all occurrences of selected word with ignoring the case.
I don't use the highlight feature. I have never understood what should be the benefit of this feature. With default search settings and option Regular expressions not checked on last regular find it is possible to select current word using Ctrl+J with default key mappings (modified by me to Alt+W) and press key F3 to find next occurrence of this word with case-sensitivity depending on Match case option as last time used on regular find. I don't understand what I should do after getting highlighted all occurrences of a word? Manually move caret to each occurrence and modify the highlighted word? No, as this can be done much faster with a replace. Just see were a variable is used with manually scrolling display if necessary? No, as this can be done better with find/find next. Perhaps I don't need this feature because I use UltraEdit/UEStudio to 99.5% with keyboard and use for that reason the existing commands for most efficiently writing and editing code and text with keyboard.
Best regards from an UC/UE/UES for Windows user from Austria