Allow split-window-sensibly to split threshold in further edge case
As a fallback, and to avoid creating a frame, split-window-sensibly would previously disregard split-height-threshold if the window to be split is the frame's root window. This change generalizes that: it disregards the threshold if the window to be split is the frame's only _usable_ window (it is either the only one, as before, or all the other windows are dedicated to some buffer and thus cannot be touched). This is required for the fix to bug#28814. * lisp/window.el (split-height-threshold): Adjust doc to match split-window-sensibly. (split-window-sensibly): Also disregard threshold if all other windows are dedicated.
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