TabBar current tab not set correctly from previous version

[kitbdev]

Tested versions

v4.3.stable.official [77dcf97], master

System information

Windows 10.0.22631 - Vulkan (Forward+) - dedicated NVIDIA GeForce RTX 2070 (NVIDIA; 31.0.15.5186) - Intel(R) Core(TM) i7-10750H CPU @ 2.60GHz (12 Threads)

Issue description

When opening a scene from a previous version with a TabBar that does not have the current_tab property set (default), the default tab for a TabBar will not be set to the first tab.

This is because TabBar::set_tab_count will set the current tab to -1 if the current tab was not set before tabs were added. This can be seen by calling the set_tab_count() manually as well.

Also related:
In TabContainer and TabBar (if there are no tabs set in the inspector), setting the current tab to an invalid value does not show error or do anything. Calling TabBar::set_tab_count afterwards will set current tab to that invalid value. It also will not emit a tab_changed signal or update the previous tab.

• The default changed in Allow no tabs to be selected in TabBar and TabContainer #87194
• The current tab was made sure to be correct in Fix error spam when adding tabs to TabBar without deselect #88494
• But it gets overridden in Tweak property order in the inspector for TabBar #88477
  This is because it assumes that TabBar::set_tab_count and TabBar::set_current_tab will be called, but they may not be.

So it looks like a regression from #88477

I am working on a comprehensive fix for this.

Steps to reproduce

• Make a TabBar in a previous version, like 4.2 with at least one tab. The current_tab property should be left as the default.
• Change version to 4.3 or later.
• The TabBar will not select the first tab by default anymore.

For setting an invalid current tab:

• Add a script to a TabBar, make sure no tabs are added in to it in the inspector.
• Set current_tab = 100 or some invalid value. No error will be shown.
• Call set_tab_count(1).
• The current tab will be the previous invalid value.

Minimal reproduction project (MRP)

tab_bar_current_tab_init_issue.zip

Three issues are shown in the MRP:

• A TabBar with valid tabs, but current tab is still -1.
• A TabContainer trying to set the current tab to an invalid value, it doesn't work but no error is shown.
• And a TabBar that does sets the current tab to an invalid value by using set_tab_count.