Try to "maximize" the Timeline view so that it effectively hides the Stage view:
Then let go of the resize handle, and try to resize the timeline view once more. You can't, the mouse cursor will no longer show the vertical resize handle.
There's a workaround which involves simply resizing (or maximizing) the SpriteBuilder window, which will then show a minimum space for the Stage View which allows you to grab the resize handle once more. This apparent minimum size of the Stage view should be adhered to while resizing the timeline view in order to avoid not being able to bring the stage view back (unless you know the workaround).
After resizing the window by even the tiniest amount the stage view pops back into view:
Try to "maximize" the Timeline view so that it effectively hides the Stage view:
Then let go of the resize handle, and try to resize the timeline view once more. You can't, the mouse cursor will no longer show the vertical resize handle.
There's a workaround which involves simply resizing (or maximizing) the SpriteBuilder window, which will then show a minimum space for the Stage View which allows you to grab the resize handle once more. This apparent minimum size of the Stage view should be adhered to while resizing the timeline view in order to avoid not being able to bring the stage view back (unless you know the workaround).
After resizing the window by even the tiniest amount the stage view pops back into view: