Problem:
- Window height is set dynamically to match the text height,
making it difficult for the user to use a different height.
- Cmdwin is closed to enter the pager but still taken into
account for the pager position, and not restored when
the pager is closed.
- Dialog pager handler may unnecessarily consume <Esc>.
Solution:
- Add maximum height config fields for each of the UI2 windows,
where a number smaller than one is a fraction of 'lines',
absolute height otherwise (i.e. `cfg.msg.pager.height = 0.5`).
- If the cmdwin will be closed to enter the pager, don't try
to position the pager above it. Re-enter the cmdwin when the
pager is closed.
- Only add vim.on_key() handler for the dialog paging is actually
possible.
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