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In graphics programs (GIMP, Photoshop, etc.) you can either drag freehand, or you can constrain the drag by holding down Shift or Ctrl. A constrained drag might, for example, maintain aspect ratio while you’re resizing a rectangle (to select, scale, crop, whatever).

In terminal or console GUIs, you drag to select text. But [very] often, you only want to select text on a single line. To do this, you have to take extra care with your mouse movements so that your drag starts and ends on the same line.

Why not take the idea from the graphics programs? Start your text selection drag anywhere you want, but then hold Shift (or whatever) to constrain the selection to a single line, regardless of whether you move your mouse cursor off the line. i.e. make the selection code consider only the x coordinate of the mouse cursor, and ignore the y coordinate.

This way, it would be easier to select text on a single line.