(trace_move) [GLYPH_DEBUG]: New variable.
(TRACE_MOVE) [GLYPH_DEBUG]: New macro. (move_it_in_display_line_to): Record iterator's ascent and descent before producing glyphs, and restore them when we know the glyph doesn't fit on the line. (move_it_to): Restructured so that it's easier to debug. If moving to a vpos, and not moving to an x or character position, stop as soon as the specified vpos is reached; don't move further into that line because that might change the computed line height. (try_cursor_movement): New function, extracted from the cursor movement branch of redisplay_window. If ending on a partially visible line, don't try to scroll if the cursor line is taller than the window. (redisplay_window): Use try_cursor_movement.
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