Sorry, my poor English makes it difficult for me to describe the problem, but I will try.
When I select text, I can click * and Obsidian will put * at the beginning and end of the text, making it italicized. I can also make the text bold or link to a page.
But this doesn’t work with == and ~~for some reason. It turns out that I can’t highlight the text and quickly make it highlighted or cross out. This is very inconvenient.
When you select text in edit mode and press * on the keyboard, the text gets wrapped by asterisks, rendering it italicized.
Based on that behavior, you expect that when you select text and press the = sign on the keyboard twice, this would wrap your selection with == and will render your selection with a highlight.
Sadly the actual result is that your text selection gets replaced by the = sign. Same goes true when you try to strike out a text selection using ~
You are frustrated, because the app’s formatting behavior is inconsistent and breaks your expectations, resulting in a poor experience.
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A suggestion for a fix may be to append and prepend all non-letter characters that get typed while text is selected.