Minor additional fix in the wake of bugs #13623 and 13626.
src/xdisp.c (redisplay_internal): Don't set w->region_showing to the marker's position. (display_line): Set w->region_showing to the value of it->region_beg_charpos, not to -1. This fixes redisplay optimization when cursor is moved up after M->. src/window.h (struct window): region_showing can no longer be negative.
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