Fix ghost space left by SplitPane with top_level=true

[boazy]

Fixes #2579, #4984

When there is more than one pane in the tab, the tab.resize() is called and the tab resize logic is reused to correctly resize all the panes in the tab before adding the new pane split, so the new split pane will not overwrite any existing pane.

The issue seems to be that after this resize, the tab "remembers" its new size, and when the new split pane is closed, the tab is left with ghost space which cannot be reused.

The fix is quite simple: we just need to immediately resize the tab back to its original proportions. Since the underlying panes have already been resized by tab.resize(), we don't need to keep the restricted tab size.

I may have missed something, but this seems to work correctly.