Win7x64hun, last Mp3tag. But, on my older computer with WinXPsp3, the situation is the same; and already some years (I can not remember, was anywhen Mp3tag without this bug or not).
Standard view with the table, details panel on the left; on the top: menu, menu buttons, filter bar. Both the table (53 columns) and the details' panel (26 data fields + the library selector) are much bigger, than in the original view. In the table there are opened 2 or 2 thousends files, MP3 or FLAC or both - no matter.
Horizontal scrollbar is displayed, of course. The thumb is about on the 20% from the left, on the scrollbar. (The scrollbar width is about 5× bigger, than the thumb's, so this is the "second click" on the scrollbar, from the left.) The cause: here displayed the "Title" column (for identifying a file in the table).
1 row is selected. I modify some data in the panel, then select the second (any) row. At the same time the previously modified file's datas will saved (by my settings). After selecting the new row, the horizontal thumb jumps to the very left (starting position) of the scrollbar. Of course, it is very annoying.
The situation repeats not every time, only about in 70-80% of cases. It happens after modifying multiple rows at once, too. The computer is not busy, resources are enough.
If somebody has this bug too, it will be nice to know about.