This tries to estimate the number of visible rows in a textview based on
the default text size and then tunes the GtkTextLineDisplayCache to keep
3*n_rows entries in the cache.
This was found imperically to be near the right cache size. In most cases,
this is less than the number of items we cache now. However, in some cases,
such as the "overview map" from GtkSourceView, it allows us to reach a
higher value such as 1000+. This is needed to keep scrolling smooth on
the larger view sizes.
With this patch, a HiDPI system with a GtkSourceView and GtkSourceMap
from the GTK 4 port can perform smooth scrolling simultaneously.
We want to ensure all of the display cache entries are released in the
normal fashion before releasing the structures so that we can maintain
the invariant that cache_iter will always exist while in the cache.
This adds a GtkTextLineDisplayCache which can be used to cache a number
of GtkTextLineDisplay (and thus, PangoLayout) while displaying a view.
It uses a GSequence to track the position of the GtkTextLineDisplay
relative to each other, a MRU to cull the least recently used display,
and and a direct hashtable to lookup display by GtkTextLine.
We only cache lines that are to be displayed (!size_only). We may want to
either create a second collection of "size_only" lines to speed that up,
or determine that it is unnecessary (which is likely the case).
