Out of interest I installed AnchorDocking and the SpartaDockedFormDesigner (I normally work without them). Removing the Align properties from the lfm does not solve anything. I also set up a new form with a bottom-aligned panel, a bottom-aligned splitter, a client-aligned DBGrid, a top-aligned DBGrid and a top-aligned splitter - no issue either. Also after adding a DBLookupCombobox and a DBListbox to the panel. But I do confirm the issue with the Lazarus dblookup example mentioned here (it has the same structure of aligned controls). Very strange...
What worries me even more, however, is that the docked form designer seems to be broken badly: create a new project, the default form is 320x240 pixels. Drag the horizontal sizer to a different width. After that, the form is displaced, incorrectly bordered and its size cannot be changed any more by dragging.
Then I wanted to change the width to 400 by typing the value into the object inspector field. After selecting the Width property I typed "4", "0", "0". However, every character typed overwrites the previous one so that the overall width becomes 0. Only when the field is double-clicked so that the entire field content is selected, the value can be typed normally.
All this is on Win10/64bit with Laz-trunk/fpc3.0.4/32 bit.
Glad that I don't use this normally...