This dropdown behavior is likely managed by a TabControl or a custom tabbed document manager within your application. Here are some areas to investigate:

1-Check TabControl Properties:

If you're using a TabControl, check if SizeMode is set to Fixed or FillToRight, as this can affect how the dropdown appears.

Look for TabControl properties like DrawMode, Padding, and Multiline that might be affecting the display.

2-Event Handling for Window Resizing:

If resizing triggers the dropdown to appear, the control might not be refreshing correctly. Look for Resize or Layout event handlers where the tab control is refreshed (Invalidate(), Refresh()).

3-ScintillaNET or Custom UI Code:

Since you’re using ScintillaNET, there might be a custom tab manager handling open documents. Check for any Scintilla or related UI event handlers that modify the tab behavior.

4-Force a Refresh When a Tab is Added:

If new tabs are being added dynamically, make sure the control is properly updated. Try manually forcing a redraw when a new tab is added:

tabControl.Invalidate();

tabControl.Update();

5-Debugging Strategy:

Set breakpoints in places where tabs are created, removed, or refreshed.

Try manually calling tabControl.Refresh() after adding tabs to see if it immediately triggers the dropdown.