It is neat and i like how it remembers the sizing and open folders, but it really shows how much Microsoft has neglected Win32 and MDI in particular: the theme is still the same used 12 years ago in Vista and while i am not a fan of the Win8 nor the Win10 theme (my favorite theme is the classic theme), the inconsistency sticks out like a sore thumb. And of course beyond the lack of a visual update (that would take perhaps 1 hour to fix), there are visual glitches like this one: https://i.imgur.com/Vfm5ndc.png (notice the black corners on the window at the right - ironically they happen because of the non-rectangular shape of the titlebar, which would not be the case if the Windows 8 or 10 themes were used since those are rectangular). And i suppose at this point it doesn't make much sense to mention missing features, like double clicking the top edge to maximize a window :-P.
Which sucks because i always liked MDI applications. I know that some studies or whatever have shown that novices often get confused, but personally i find the idea of having a "group" of windows inside the application that manage them to make more sense - especially for visual stuff, like image editors where i can open two views of the same document in different zoom levels. And i also find being able to move and resize the subwindows in any way i want (not limited to splits and such which i find more annoying and a waste of space than helpful).
Having said that i always always disliked, from the first day i saw it, the visual change from Windows 3.1 to Windows 95 to make minimized windows look like tiny titlebars (they were called buttons but there is nothing button-y about how they ever looked in any version of Windows from 95 to today) instead of icons. mIRC adding a taskbar-like panel was a much better idea (you could say that novices would again be confused, but from what i remember from the late 90s and early 2000s, that didn't stop people from using mIRC :-P), but just sticking with icons would be fine too (i think icons inside the MDI area were supposed to be used for something else that i don't remember now, but i do not think any program ever used that).
EDIT: added a tiny feature to search multiple wildcards separated by semicolons :-) - https://i.imgur.com/vi7BI7B.png
The code style is kinda weird though (3 spaces for each indentation level?) and seems that several comments are out of date :-P.