new post content starting new line half way through

I’m not able to reproduce that on my end, but a quick way to tell if it’s a theme issue or not is to switch to a default theme temporarily – like Twenty Fourteen or Twenty Thirteen – to see if the behaviour is the same. You may need to reset your widget and other theme-specific settings afterwards, so you might want to make a note of them.

Another troubleshooting technique is to try deactivating all your plugins. Does the problem go away? If yes, turn the plugins back on one-by-one to find the conflict.

One more idea: have you tried a different browser? If it happens only in one web browser (like Firefox or Chrome) then try deactivating all of that browsers add-ons or extensions. If the problem goes away, then you know an add-on is to blame and you can try turning them back on one by one to determine which one is at fault.

Finally, clearing your browser’s cache often fixes problems like this, which are sometimes caused by incompletely loaded file in the editor.

Let me know how it goes!

Thanks for all your suggestions!

I changed to a couple of other themes and have mixed outcomes. Some show the line properly, some compact it. Most use the full length.

Plugins checked. No difference.

Tried different browser and same thing.

I changed to a couple of other themes and have mixed outcomes. Some show the line properly, some compact it. Most use the full length.

I think I understand better what you mean now!

Some themes apply the content width (the width of the main column in your theme) from the theme’s stylesheet to the post editor (where you write your posts), while others don’t.

It’s meant to give you a better indication of how the text will look on the front end (public side) of your site – for example, in terms of line length, once the post is published.

Let me know if this answers your question!