which is LButtonUp and LbuttomDown? when click at 公式 after， and press delete，I can not reappear.

Left button up/down at mouse.

Click, enter, click again, delete please

or move the cursor key the front of a, enter, move the cursor back, and delete

click , del, del, del, del, del, del... (can not delete)
ctrl+z, ctrl+z, ctrl+z, ctrl+z.....

Edit inline/block math and code should move caret into its content other than in render result:

Undo related issues we will fix in #85 thank you!

I think all text should be erased with only one backspace. try it from bottom.