There is a problem with the Sparta docked form designer and inherited forms. Since some of you might not know you can easily register custom inherited forms with Lazarus I made a brief video going over the problem:

http://cache.getlazarus.org/videos/docked-designer-issue.mp4

I am producing a few professional grade Lazarus tutorial videos and ran into this road block. This issue needs to be fixed. I'll look into a fix, but maybe Maciej or some others can help.

Mantis issue #29615
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2016-02-07 20:34 GMT+01:00 Anthony Walter :

the solution is probably not that simple. Maybe different way of handling TForm is needed.

I was looking at the code. There might not be an easy solution but the docked designer really needs to support this feature. Can you point out where in the package you are picking the class for and then creating the docked design control?
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2016-02-07 23:00 GMT+01:00 Anthony Walter :

Sparta changes are deep. All is in IdeIntf
package (DesignerBaseClassId_TForm const is interesting for us. You can try to search DesignerBaseClassId_TForm in whole ide src). Maybe somehow in module ide/SourceFileManager.pas this kind of form (reported in issue #29615) is wrongly handled (just a guess). Strange bug, even non LCL forms demo works correctly with Sparta (see examples\designnonlcl).

In your case bug fix can be trivial or very complicated. Sooner or later I will fix it. Anyway you can try on own hand :). For now I need to focus on custom Sparta MDI solution and on deep refactoring.