The maximized window isn't really maximized. I can see my desktop wallpaper at the bottom. Compare to Notepad, where I can not see part of the wallpaper, just part of the auto-hiding taskbar:
vs.
CCleaner should just use native windows elements and not its own custom elements.
On a side note, certain visual elements are still not DPI aware, like v4. See the pixelated check marks and icons.
Just thought I'd revisit this thread. There is a new maximize issue in CCleaner 5.09.5343. See screen shot.
There is blank space at the left, bottom and right of the "maximized" window. In the background there is a maximized Notepad. If I manually try to resize CCleaner, I cannot make it the entire width of the screen either. This last part happens with some other applications as well, for example Windows Media Player...
I've also noticed that the smaller icon at the very top left is still somewhat pixelated, the rest looks great with Windows 10's new scaling, set at 125%. Though the installer has the fuzzy text issue, which doesn't really bother me, but is apparently a hot topic for Windows 10.
Finally, I noticed the title bar now uses native elements for the buttons, this is great! Can't wait for a future Windows 10 update to give me the option to color the title bar...