Probably, this has already been fixed in bug 1724650 from point of view of Core product. The remaining issue is probably that mail composer does not specify `white-space: pre-wrap` to the body. However, I'm not sure whether the style is available in any major web-apps. Therefore, it's out of scope of this bug that whether specifying the style **and** exposes it as-is or converting only collapsible white-spaces to NBSPs or not at sending email it. I'll add automated tests for avoiding regressions in the Core product.
Bug 532712 Comment 26 Edit History
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Probably, this has already been fixed in bug 1724650 from point of view of Core product. The remaining issue is probably that mail composer does not specify `white-space: pre-wrap` to the body. However, I'm not sure whether the style is available in any major mail-apps. Therefore, it's out of scope of this bug that whether specifying the style **and** exposes it as-is or converting only collapsible white-spaces to NBSPs or not at sending email it. I'll add automated tests for avoiding regressions in the Core product.