[BUG] MDI window controls (Minimize/Maximize/Close) are completely hidden unless hovered

Description

This may work as intended, but it causes erroneous closing of the whole application if one is not careful:

In the current MDI interface, when a file comparison window is opened inside the main folder comparison view, the window controls (Minimize, Maximize, and the Close 'X' button) are completely invisible until the user hovers their mouse directly over the exact area where they are supposed to be. There's really no reason to hide them. There's a ton of icons to the left. Hiding these buttons leads to closing the whole app by mistake. The window control buttons should remain visible at all times, as in regular windows.

Steps to reproduce

No response

WinMerge version

2.16.58

Platform

Windows x64

Additional information

No response