Verified rendered, both widths, on a real machine (ConfRoomROG, Parts Search 2.0.13, 2x display). Two screenshots attached, cropped to the search bar.
Wide (1263 CSS px): the full placeholder shows with a clean gap before the per vendor control. No clipping, no collision. In your first screenshot it was chopped at STM32F103RBT6 and jammed against the control.
Narrow (607 CSS px, below the 720px breakpoint): the magnifier now sits on the same row as the input, then per vendor and the toggle stack beneath, then the full-width button. In your second screenshot the icon was orphaned alone on the first row with the input dropped underneath it. That jump was also most of the "startling" resize.
Root causes, for the record: the input was forced to order:1; flex-basis:100% at the breakpoint but the icon had no order, so it stayed on line one by itself (fixed by making icon + input one flex unit). And the placeholder was 66 characters with no text-overflow, so it clipped mid-word (fixed with an ellipsis, and shortened since the Try chips below already list the examples).
Closing. If it still misbehaves at a width I did not test, reopen with the width and I will take it from there.

