Firefox bug with negative margins about imscjs

[nigelmegitt]

Noting for posterity: the Firefox bug was raised almost exactly 5 years ago in October 2018; it only appears in imscJS when the per-character adjacent spans are merged together, which has other rendering benefits in some engines. The span merging was introduced in #216, in 2021.

An alternative potential fix is to avoid merging the span of the last character in the line, if using Firefox.

One way to implement that could be to setting a variable to 0 if not Firefox and 1 if Firefox, and subtracting that value from line.elements.length at

imscJS/src/main/js/html.js

Line 593 in 32d0767

for (var j = 1; j < line.elements.length;) {

for example. (I have not tested that this specific change results in the behaviour I'm expecting)