In 1.11.2 the enchantment GUI's book animation opened and closed smoothly, at the the same framerate as the game was running. In 1.12.2 (and in snapshot 17w45a) the animation jumps between positions, staying in a single one for a few frames.

From the equivalent forge code (1.12.2, MCP data version "snapshot_20170629") it appears that minecraft calls GuiScreen.drawScreen with a parameter for partialTicks with Minecraft.getTickLength() rather than Minecraft.getRenderPartialTicks(). (The former is the amount of time, in ticks, from when the last frame was rendered, the latter is the amount of time, in ticks, since the last tick). Note that this is reproducible without forge, so it is not caused by a broken forge patch, but it is more obvious in modded GUI's that have larger animated elements than the enchanting table.