On Windows 10, paint.net assumes the inactive window titlebar color is white, and draws its window title string with that background color. The inactive titlebar color should be computed by checking for two registry keys in HKCU\SOFTWARE\Microsoft\Windows\DWM\: if ColorPrevalence is set to 1, and AccentColorInactive exists, the latter key holds the inactive window titlebar color. If either of these fail, the inactive window titlebar color is white.
Attached is a screenshot of how paint.net appears when inactive on my system (where these keys are set).
