TTabSet tab Order…Different At Run Time Than Design Time

Question

I am running Lazarus 0.9.30.2.

I have a TForm on which there is a TPageControl. Within the TPageControl there is a series of TTabSheets. At runtime the order of the TTabSheets differs from design time (see picture).

The order in design time is what I want to see at runtime, at least for the very first time the form is displayed. Why does the order change at run time and is there a way to control this?

Answer 1

@TLama is correct that this is related to way the Windows tab control behaves when in multi-line view. The behaviour you are observing is related to the way selection is handled for multi-line tabs. When you select a tab it is always shown in the bottom row because the visual cue to indicate which tab is selected can only really work for tabs in the bottom row.

Given that constraint the control simply has to rearrange rows of tabs as you modify the selected tab. It's astoundingly confusing for the user. Good UI design never has UI elements changing position like this.

Clearly what is happening here is that the rearrangement is happening at runtime when the form is first shown and for whatever reason this is resulting in a different arrangement from the design time arrangement. Given that the user can arrange the rows in any order just by selecting them I'm not sure you should worry about what order the rows appear in.

If you are dead set on forcing a particular arrangement when the form first shows you can add code like this to a OnCreate handler for the form:

PageControl1.ActivePage := TabSheet9;
PageControl1.ActivePage := TabSheet5;
PageControl1.ActivePage := TabSheet1;

Best practise for UI design is to avoid multi-line tab controls and I urge you to attempt to re-design your UI that way.