Windows 10 (and likely other Windows versions)
OpenCV 4.x and 3.x

C++ code to reproduce:

#include "opencv2/highgui.hpp"

int main()
{
    std::string const window_name{ "TrackBars" };
    cv::namedWindow(window_name, cv::WINDOW_NORMAL);
    cv::resizeWindow(window_name, {500, 200 });
    cv::createTrackbar("Slider 1", window_name, nullptr, 255);
    cv::createTrackbar("Slider 2", window_name, nullptr, 255);
    cv::createTrackbar("Slider 3", window_name, nullptr, 255);
    cv::createTrackbar("Slider 4", window_name, nullptr, 255);
    cv::waitKey();
    return 0;
}

Run that...

resize the window so it's wide enough to hold 2 trackbars side-by-side -- all good ..

and then resize the window so it's wide enough to hold 3 trackbars side-by-side.

The problem is in the WM_NCCALCSIZE message handler in HGToolbarProc, specifically in line 2072:

With 4 toolbars as an example scenario, when going from 2 to 3 columns, the number of rows stays the same, which means that the trackbar and buddy control windows don't get re-positioned. Since the idea is for those two windows to completely cover the placeholder buttons, and they didn't get re-positioned, we get to see the placeholder.

I think the easiest way to deal with this is just get rid of the if and have it re-position the controls one each call. It seems to behave fine like that, and the overhead doesn't seem noticeable even in debug mode.

One more thing I've noticed -- that SendMessage on line 2074:
SendMessage(window.toolbar.toolbar, TB_BUTTONCOUNT, 0, 0);
seems rather pointless, given that the result is ignored. Unless this is meant to have some unexplained side-effect, I'd propose to get rid of it.

It appears to be 13+ year old piece of code. Removing it doesn't appear to have any negative effects on my system.