If you have a horizontal scroll bar on the message pane, then you are using a small screen device, or are using a fairly wide folder pane.

Starting in the message pane (in classic - bottom right window), try moving your mouse slowly up the screen towards the thread pane. At the junction of the thread pane and the message pane there is usually a horizontal line, but your screen may show a horizontal scroll bar for the upper window instead. Your cursor should change to a double-headed arrow at the transition point. Left-click and drag the double-headed cursor up or down to resize.

TB38.2 Win10

I am using a 19" monitor, and have a normal folder pane on the left. This problem was there for several days. No amount of slow mousing would enable the transition point. Nor would changing the folder width etc. Or view changes.

I just now went through several more shutdowns, and many view changes. Suddenly, the slide bar just disappeared a few minutes ago, and both the thread pane and the top separator line reappeared.

No idea why.

Sometimes computer problems can be corrected by simply closing and reopening the program, or by restarting the computer. Scares the gremlins away.

Can we assume that TB is working, for now?

It's working now. Never seen anything like that before. Took 3-4 days, & a dozen or more shutdowns.

Oh well......