Add "focus-follows-mouse"-like option but for outputs only

[xezo360hye]

I like how "focus follows mouse" works in general but it's not something I could normally use here with two monitors placed side-by-side since moving mouse to the other screen can move the windows view frame. Therefore I have disabled it, but it's somewhat annoying that I need to click to focus even when going with mouse to the other monitor. I wish there was an option like "focus-follows-mouse" but exclusively for outputs.

Since focused window on each monitor is already stored separately (I assume so since there exists "focus-ring") it shouldn't require a lot of changes in existing code I guess. A general algorithm is the following:

1. For each mouse movement event
2. Check whether the pointer has moved off screen
3. Set the screen under cursor as focused
4. Set last focused window on it as currently focused

Didn't look into the code yet, maybe the 4th step isn't needed at all. I'd try to implement it myself but I don't have enough time for it as of now, and at least not until exams are over.

[YaLTeR]

Hmm, maybe, yeah. You're right, every monitor (every workspace actually) stores a focused window, there's no need to do anything extra.

[tv42]

Somewhat related, I would love a focus-follows-mouse that changes focus (even within one output), but never changes the layout. On niri 0.1.6, if you have a screen that shows

<--screen width-->
| 1/2 col | full width col |

and you move the pointer from the left column to the right column, the screen scrolls to only show the right column.

I don't want scrolling to happen just because I'm moving the pointer, but I can't live without focus-follows-mouse.

[tv42]

Yes. I need focus-follows-mouse but IMHO it's jolting to see the windows jump around from just mouse movement.

Consider a floating window manager: if I move a window partially off-screen, it doesn't jump into view when I hover over it.

[YaLTeR]

Well, in a floating window manager you don't frequently have windows mostly off-screen as part of the normal use.

[tv42]

Here's another scenario in which the windows moving around bothers me:

A monitor is on the right of the laptop. The monitor has a leftmost column that is not fully visible. I've been interacting with the two 1/3rd width columns, and want to leave them visible to keep reading their content. I move the mouse pointer from the middle column of the monitor to the laptop display, to write notes there.

The current Niri will scroll the columns on the monitor, making only the full width window visible, hiding the windows I wanted to refer to.

If I could disable this automatic scrolling, the columns on the monitor would remain visible.

<--laptop-->
| whatever |
            <-----------monitor----------->
 |..   full width | one third | one third |

[xezo360hye]

Here's another scenario in which the windows moving around bothers me:

A monitor is on the right of the laptop. The monitor has a leftmost column that is not fully visible. I've been interacting with the two 1/3rd width columns, and want to leave them visible to keep reading their content. I move the mouse pointer from the middle column of the monitor to the laptop display, to write notes there.

In fact you just described my exact setup and the reason I created this issue.

Btw I'd appreciate if someone could point me to the relevant files/directories of the project now that I have a bit more time.

[YaLTeR]

The config preference:

niri/niri-config/src/lib.rs

Line 79 in 22a948c

pub focus_follows_mouse: bool,

Since we need to retain backwards compatibility, it should still work as on/off by itself, so maybe a good way would be to add a property to it, i.e. focus-follows-mouse outputs-only=true. This may need wrangling with knuffel to do custom parsing. There are various custom parsing examples with other parts of the config.

The logic itself: should be trivially changeable here

niri/src/niri.rs

Line 3828 in 22a948c

if let Some(window) = &new_focus.window {

[YaLTeR]

Let's discuss in #491