Root cause of 'only the cool/blue knob drags, regardless of where you
tap': LVGL's default arc hit-testing just uses each widget's full
bounding square (whichever topmost widget's box contains the touch
point wins), not the precise ring geometry. Since cool_arc's 360x360
box is entirely contained within heat_arc's 400x400 box, cool_arc (on
top in z-order) claimed every touch inside its own square, including
touches physically on heat_arc's outer ring, which sits outside
cool_arc's box but still within heat_arc's simple bounding-box test.
Confirmed via lv_arc.c: LVGL has a precise ring-based hit test
(LV_EVENT_HIT_TEST, respects each widget's own radius and angular
span) but it's opt-in via the ADV_HITTEST flag; without it, LVGL
falls back to the simple bounding-box test. Verified the adv_hittest:
true YAML property generates lv_obj_add_flag(..., LV_OBJ_FLAG_ADV_HITTEST)
via an isolated test compile before applying to both arcs.
The differing radii (400/360) from the previous fix are still useful
for visual separation but were never sufficient on their own — this
flag is what actually makes hit-testing respect the radius difference.
pyr0ball
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