clearstream
(He, Him)
I’m not sure what you’re getting at about choosing a number of rounds of combat... The number of rounds it would take to add up to an hour is 600, since each round is 6 seconds. In other words, combat alone just isn’t going to cause the PCs to miss out on the benefits of a long rest, and I believe that is the design intent.
I think that’s the intent. Having your long rest interrupted by a 3-5 round combat (or heck, even several 3-5 round combats) won’t be a significant enough interruption to prevent you from gaining the benefits of the rest. It’ll just tax your presumably already depleted resources even more, and probably be more dangerous than your typical combat since most of the party will probably be in their long johns for the fight. 5e really doesn’t want late-night monster attacks preventing the party from getting their resources back. To really screw up a long rest, you need to spend significant time not resting.
From your earlier post, it sounded like you had in mind some kind of bar for how much combat "won't be significant" with the implication that more than that amount could be significant. It sounded like you had in mind up to several average combats: a few minutes of fighting.
However, I now understand that you mean that any amount of combat is fine. For me that is jarring. Noting the comma after the word "walking" - as in "1 hour of walking, fighting, casting spells, or similar adventuring activity" - and with everyone I have discussed this with agreeing that 1 hour of fighting, let alone 1 hour of casting spells is implausible, it seems more reasonable to understand that it is a separated list being either 1 hour of walking, or any amount of fighting, or any amount of casting spells, or any similar adventuring activity.
To read it the other way is to ignore cues in the language and problems of reasonableness, in order to sustain the way one wishes to play. Essentially one says - because I don't want combat to interrupt rests, I will assume that comma is not acting as a separation but a conflation. The residual problem is then it must also be 1 hour of casting spells... which on surface sounds preposterous! It is not the most reasonable position to call that "as intended".