Fix bug #18545 with lack of scrolling a window when point goes out of view.
src/xdisp.c (cursor_row_fully_visible_p): Update commentary. (redisplay_window): Treat the frame's frozen_window_starts flag the same way as the optional_new_start flag for the window: only obey it if the glyph row showing point will be fully visible. Likewise when the window start is in a continuation line. If, after trying everything under the 'force_start' label, point is still not fully visible, give up and scroll the window. Add debugging traces. src/window.c (Frecenter): Set the window's redisplay flag.
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