MacOS Terminal.app can’t split with Command + D

I finally found the solution to this problem here: https://coderwall.com/p/h6yfda. The necessary steps are shown below:

1. Go to Preferences, Profile, Keys.
2. Set your left ⌥ key to act as an escape character.
3. Locate the current shortcut for ⌥ ← or create a new one, with the following settings:
   • Keyboard Shortcut: ⌥←
   • Action: Send Escape Sequence
   • Esc+: b
4. repeat for the ⌥→ keyboard shortcut with the following settings:
   • Keyboard Shortcut: ⌥→
   • Action: Send Escape Sequence
   • Esc+: f

It's the history -a part of PROMPT_COMMAND in your .bashrc - that's appending to the history file every time a prompt is displayed, whatever tab you're using. A quick fix would be to remove this, so that each tab's session only writes to .bash_history when the session ends (e.g. when the tab is closed).

I like this behaviour (immediate appends to .bash_history), and I suspect you may too! A better solution may be to maintain separate histories for each session/tab - one of the answers to this question describes how to achieve that. You could retain immediate appends, but to separate files - one for each session. You would still be able to search your entire bash history with a command like grep "foo" ~/.bash_hist*. (I've not personally tried this: I use one history file for all sessions, since my normal workflow tends to be in one tab or through screen, and I archive my /bash_history periodically into a separate file, as described in this article - which is well worth reading, even if it doesn't address your issue).