Weird corruption when using Metrics View in compact CID-keyed fonts

[ctrlcctrlv]

Necessary file: 蒙纳繁勘亭流.otf (CKanHK-XboldSupp)

This bug takes the cake for bizarre CID bugs.

Notice how glyphs are being drawn on top of FontForge's gray X which in normal operation is supposed to indicate, there's no glyph here. If we open the corresponding CharView of these glyphs we get an empty window.

Here's how we get ourselves into this bizarre broken state:

1. Open the necessary file
2. CID→CKanHK-XboldSupp
3. Critical step: Encoding→Compact
4. Metrics View
5. Enter text 何が起こってる？
6. Back to FontView, find こ. Now corrupt.

Much more “fun” with this corruption is possible. Let's say we skip the Compact step, and do it as normal.

Then let's hit Encoding→Compact! Be sure to press space! Instant corruption.

Let's go to the FontView, «Ctrl→>» (Goto) and enter ‹こ›…

It obviously should not be possible to have two ‹こ› in one file. Our memory seems hopelessly corrupt.