So, this seems to be a strange bug with mouse controls, but (un)fortunatly i got it not just in one save, but in all my saves (1.12.2 forge) (maybe 107 mods has something to do with this) and so figured out some stuff:
1) Bug (in my case) works like this: i click on any rideable creature, camera swings in a random direction, but always stays exactly horisontal, and nothing else happens
2) If i press shift, it works fine some times, but i`m pressing shift, so no luck
3) The direction of camera movement is the same, until horse moves, then its other direction
4) [Solution]: rmb on a horse, position yourself, so when you click next time your cursor would stay on horse (the horse is a little bit below the 'cursor horison', so you must be one (or half (or fraction of)) block below, click again => profit.
5) The bug seems to happen, when you click on a horse and normally it would reset camera position after you embark, but it instead resets first, and then tries to check, what you clicked again (one time it opened a chest (mailbox, actually) after reseting the camera) and if it is not a horse, no luck then. Shift 'works' becouse it makes your 'cursor horison' a bit closer to the ground => closer to the horse (enough to hit)
6) [Better solution] By using a shovel on grass you make a pathway. A pathway makes you low enough to hit a horse, so does the soul sand (i think)
Nymda
So, this seems to be a strange bug with mouse controls, but (un)fortunatly i got it not just in one save, but in all my saves (1.12.2 forge) (maybe 107 mods has something to do with this) and so figured out some stuff:
1) Bug (in my case) works like this: i click on any rideable creature, camera swings in a random direction, but always stays exactly horisontal, and nothing else happens
2) If i press shift, it works fine some times, but i`m pressing shift, so no luck
3) The direction of camera movement is the same, until horse moves, then its other direction
4) [Solution]: rmb on a horse, position yourself, so when you click next time your cursor would stay on horse (the horse is a little bit below the 'cursor horison', so you must be one (or half (or fraction of)) block below, click again => profit.
5) The bug seems to happen, when you click on a horse and normally it would reset camera position after you embark, but it instead resets first, and then tries to check, what you clicked again (one time it opened a chest (mailbox, actually) after reseting the camera) and if it is not a horse, no luck then. Shift 'works' becouse it makes your 'cursor horison' a bit closer to the ground => closer to the horse (enough to hit)
6) [Better solution] By using a shovel on grass you make a pathway. A pathway makes you low enough to hit a horse, so does the soul sand (i think)
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Blesh you in your fight against rare bugs 🙂