ligatures drawn imperfectly

i tried out the macligatures feature. with muliple fonts is see this:

1. typing a != and leaving the cursor in insert mode, a somewhat broken 'not equal' ligature appears. when moving the cursor to the next (empty) line, this is repaired.

2. moving the cursor from the = to the next empty line and back again a few times again reproduces the broken ligature on screen. moving the cursor further down to a nonempty line then repairs it.

3. generally, moving the cursor onto one part of a ligature redraws the exact cursor position with the non-ligated glyph, but the adjacent positions retain chopped parts of the ligature.

it seems the problems stem from a too-local redrawing rule? better would be to redraw all characters of a ligature together, or possibly un-ligate the whole line of the cursor always.

this is the behavior with the Core renderer. with the non-core renderer i see slightly different problems. in particular when the cursor is on = the first part of the ligature stays black.

cursorline and relativenumbers disabled. filetype was python.

this exact behavior is with Fira, other fonts gave similar results. macvim 8 patch 329.

Reproduce the a != case with Fira using the Core renderer, then compare it with the non-Core renderer while moving the cursor as described. Trace the redraw behavior around the cursor and ligature boundaries; done means both renderers consistently redraw the complete ligature without chopped or stale glyphs.