When a line contains a character and a zero-width joiner (ZWJ) in that order, selecting the character by hand, deactivating the page by putting Safari in the background, causes the selection to extend to the ZWJ. Moreover, activating the page by bringing Safari to the foreground and clicking somewhere else in the document to deselect the text may cause the character to be deselected and the ZWJ to remain selected.
Created attachment 331980 [details] Tests
The extending of the selection highlight to include the zero-width joiner is a regression from the selection highlight behavior seen in shipping Safari version 10.1.2 (12603.3.8) on macOS Sierra (16G1004).
(In reply to Daniel Bates from comment #2) > The extending of the selection highlight to include the zero-width joiner is > a regression from the selection highlight behavior seen in shipping Safari > version 10.1.2 (12603.3.8) on macOS Sierra (16G1004). Elaborating further, this bug is specific to macOS High Sierra. That is, it does not reproduce on macOS Sierra.
<rdar://problem/36799695>