Where can I find the "Horizontal Splitter" in Apple Mail?

The lack of that visual feedback runs counter to Apple's own Human Interface Guidelines: https://developer.apple.com/library/content/documentation/UserExperience/Concept ual/OSXHIGuidelines/Feedback.html#//appl…

Instantly acknowledge the user’s commands and input. Users expect to receive some type of feedback every time they interact with your app. In macOS, buttons respond visually when users click them, and the pointer changes appropriately as users move it over different controls and areas. Similarly, if a command can’t be carried out, users want to know why it can’t and what they can do instead. The quicker you provide feedback for the user’s interactions, the more responsive your app appears.

Perhaps one day they will follow it.

Refer to the rightmost part of the window, where you last dragged the left pane all the way to the right. When positioning your cursor over it, the normal selection cursor will change and bear a resemblance to this

<-|

Drag that.

Drag that cursor to the left. Drag it farther than you have been doing, much farther. There will be no visual confirmation of that action until you drag it more than halfway to the opposite edge of the window, or full screen if you are not using windowed mode.

For the horizontal splitter: grab the bottom boundary (the little grey dot) and move it upwards.

Same for the right boundary: move it to he left.