I'm developing a rhythm game for iOS which has four buttons spanning the width of the screen in portrait. I noticed that my testers were having some missed inputs on the buttons on the left and right due to the fact that iOS, by default, tries to ignore accidental touches on the edges of the screen. So I enabled "Defer System Gestures" on the left and right edges, but then quickly started to notice a new, very specific, issue.
Description of the issue
If you have finger #1 touching and holding anywhere in the middle of the screen, and finger #2 touches on the far right or left edge of the screen just below the horizontal position of finger #1, those touches are inconsistently not recognized. If finger #1 is not present, this issue does not occur. If finger #2 is above or well below finger #1, this issue also does not occur. A dead zone is created on the right and left edges of the screen just below the horizontal position of the first touch.
Here is a rough representative example of where touches #1 and #2 need to be for this issue to manifest, in case the text above is not clear.
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It just so happens that this issue is causing major usability problems with my game, as it results in what the user sees as sporadic and inconsistent response when the game calls for two notes to be played at the same time.
Steps to recreate the issue
Here are the steps if you want to recreate the problem yourself using the "Create New Gesture" pane in "Assistive Touch" (Note that this problem is not specific to the Settings app, but rather is an issue across the system—however this panel defers system gestures and shows where touches are being read, so it is a great place to demonstrate):
(1) Go to Settings > Accessibility > Touch > Assistive Touch > Create New Gesture...;
(2) With one finger, touch the middle of the screen and hold it through step 3;
(3) With a second finger, tap 4 times along the right (or left) edge of the screen in the following places:
(a) well above the vertical position of the first touch,
(b) just above the vertical position of the first touch,
(c) just below the vertical position of the first touch, and
(d) well below the vertical position of the first touch;
(4) Notice how, more than half the time, touch (c) does not register. I have found that this problem is more replicatable when the first touch is on the lower half of the screen, but I have been able to replicate it when the finger is higher as well, just not as consistently.
Here are the four positions described in the steps above:
Position a: both touches register
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Position b: both touches usually register
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|	 1	 2|
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Position c: only touch 1 registers
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|	 1		|
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Position d: both touches register
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|	 1		|
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|			 2|
Is there anything I can do to resolve this behavior? My app requires gesture deferment to be on for the expected experience by the user, and this bug is causing other issues for my testers that kind of need to be resolved before I can confidently release the game.