Smoother scrolling in the terminal — a feature decades in the making¶
The great philosopher F. Bueller once said “Life moves pretty fast. If you don't stop and look around once in a while, you could miss it.”
Beuller was not taking about terminals, which tend not to move very fast at all. Until they do. From time to time terminals acquire new abilities after a long plateau. We are now seeing a kind of punctuated evolution in terminals which makes things possible that just weren't feasible a short time ago.
I want to talk about one such feature, which I believe has been decades1 in the making. Take a look at the following screen recording (taken from a TUI running in the terminal):
Note how the mouse pointer moves relatively smoothly, but the scrollbar jumps with a jerky motion.
This happens because the terminal reports the mouse coordinates in cells (a cell is the dimensions of a single latin-alphabet character). In other words, the app knows only which cell is under the pointer. It isn't granular enough to know where the pointer is within a cell.
Until recently terminal apps couldn't do any better. More granular mouse reporting is possible in the terminal; write the required escape sequence and mouse coordinates are reported in pixels rather than cells.
So why haven't TUIs been using this?
The problem is that pixel coordinates are pretty much useless in TUIs unless we have some way of translating between pixel and cell coordinates. Without that, we can never know which cell the user clicked on.
It's a trivial calculation, but we are missing a vital piece of information; the size of the terminal window in pixels. If we had that, we could divide the pixel dimensions by the cell dimensions to calculate the pixels per cell. Divide pixel coordinates by pixels per cell and we have cell coordinates.
But the terminal reports its size in cells, and not pixels. So we can't use granular mouse coordinates.
What did people use pixel coordinate for?
This does make we wonder what pixel reporting was ever used for in terminals. Ping me on Discord if you know!
At least we couldn't until this recent extension which reports the size of the terminal in cell and pixel coordinates. Once we have both the mouse coordinates in pixels and the dimensions of the terminal in pixels, we can implement much smoother scrolling.
Let's see how this looks.
On the left we have the default scrolling, on the right, Textual is using granular mouse coordinates.
| Default scrolling | Smooth scrolling |
|---|---|
|
 |
Notice how much smoother the motion of the table is, now that it tracks the mouse cursor more accurately.
If you have one of the terminals which support this feature2, and at least Textual 2.0.0 you will be able to see this in action.
I think Textual may be the first library to implement this. Let me know, if you have encountered any non-Textual TUI app which implements this kind of smooth scrolling.
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