For a few weeks now,
I’ve noticed an oddity when typing within a Forums text input field…
When I want to replace text,
I generally highlight the text and then type.
The new text ordinarily should automatically replace what was highlighted.
Nowadays,
Typing does not over-write, the highlight text is immutable.
Instead,
I have to delete (or backspace) the highlighted text to remove,
and even after that backspace or left-arrow to move the cursor
before I can type the replacement text.
This idiosyncrasy is not seen anywhere else, so is specific to the Forums software.
I can’t reproduce what you are experiencing. I’m using Chrome Version 65.0.3325.162 (Official Build) (64-bit). Below I’m going to write the same thing on two different lines, then go back and replace a few words in the first line:
The quick brown fox I REPLACED THE WORD JUMP HERE over the lazy dog’s back
The quick brown fox jumped over the lazy dog’s back
I’m now using Firefox 58.0.2 (64-bit) to post this and it works the same as Chrome:
The quick brown fox jumped over the lazy dog’s back.
The quick THIS HAS BEEN REPLACED AFTER THE FACT over the lazy dog’s back.
The quick brown fox jumped over the lazy dog’s back.
The quick brown fox jumped over the lazy dog’s back.
Not a problem here, perhaps because I am using “firefox”.
I did have problems when I used “vivaldi” (which is chromium based). I think it has to do with javascript.
When you run into the problem, try clicking the “A/A” near top left of the editor box. That switches to a different edit mode that uses less javascript.