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Re:Trinity Desktop Environment R14.0.9 Release Notes by emilsedgh on Monday November 02, 2020 @04:09AM (#24966035)
What you're describing for a "dbus but for ui" is one of the core KDE technologies still in use today named KPart [0][1].

[0] https://techbase.kde.org/Development/Tutorials/Using_KParts

[1] https://en.wikipedia.org/wiki/KDE_Platform_4#KParts

Re:Trinity Desktop Environment R14.0.9 Release Notes by badsectoracula on Tuesday November 03, 2020 @07:45PM (#24985686)
Yeah i know about KParts, however KParts is more limited and the tech is only about KParts whereas COM is used for a lot of things, including even stuff unrelated to the GUI (e.g. Active Scripting). In addition COM defines a binary interface for accessing objects that can be supported by any programming language that can support structs and pointers whereas KParts is only for C++.