Floating windows: keep KWin's placement memory out

KWin's PlacementTracker keeps each window's geometry, full screen and
maximized state per layout of the outputs, and puts windows back when a
layout it has seen comes back. A spare output resizes after its window,
so resizing a floating window back to an earlier size (or changing its
scale, or full screen) made the window and its output flip forever, and
floating or docking one window could move others onto or off a spare.

The KWin script now keeps where each window belongs, reports nothing
while KWin changes the outputs, and on screensChanged puts floating
windows back (and the screens' windows when only spares changed),
cancelling KWin's requests before the app sees them. A size asked for is
held for a second against late answers.

With that: a launched app and a profile get their remembered scale back,
scale steps keep the size in pixels, ft-floatd waits for the end of an
edge resize before resizing the output (KWin cancels the resize on any
output change), a window taken over after a restart keeps its app and
panel density, and `ft-float float ID` asks the script for the window's
current place.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/floating-windows.md

@@ -11,14 +11,13 @@ Built so far (2026-09-29; the KWin side tested on the headless test desktop, `sc
 - The 3D mouse's drag lock crosses onto other Frametop panels (not while carrying one).
 - The float key (2026-09-30): the input relay's `float_toggle` (Meta+Shift+F by default) and `dock_all`, through `float pointer` and `dock all` on ft-floatd. Tested on the headless desktop.
 - The title bar button (2026-09-30, branch `float-titlebar`): `decoration/` (Frametop's QML window decoration, installed by the session script; `decoration/apply.sh` switches the running desktop to it or back to Breeze), and keep-below as the floating flag in the script. Tried on the live desktop: the button floats Dolphin and docks it again, the float key and `dock all` keep the flag in step, and maximized windows look right.
-- Launching floating (2026-09-30, branch `float-launch`): `ft-float launch APP` and `ft-float run COMMAND`, matching the new window by process (or a child) or by desktop file name for 30 seconds, and each app's remembered place (pose relative to the primary screen, size in pixels, scale) in `~/.config/frametop-float.json`, kept whenever one of its windows stops floating. With nothing remembered, the window opens in front of you, at the primary screen's density, 0.8 to 2 m away. Launch as Standalone (`float/ft_apps.py`): the session writes the desktop file copies and puts them first in `XDG_DATA_DIRS`; ft-floatd rewrites them when apps change. Tried on the live desktop: Dolphin launched floating, came back at its remembered pose after closing, and Konsole floated from `ft-float run`; the right-click entry needs a desktop start to show. The remembered scale is kept but not applied yet (see Known problems).
+- Launching floating (2026-09-30, branch `float-launch`): `ft-float launch APP` and `ft-float run COMMAND`, matching the new window by process (or a child) or by desktop file name for 30 seconds, and each app's remembered place (pose relative to the primary screen, size in pixels, scale) in `~/.config/frametop-float.json`, kept whenever one of its windows stops floating. With nothing remembered, the window opens in front of you, at the primary screen's density, 0.8 to 2 m away. Launch as Standalone (`float/ft_apps.py`): the session writes the desktop file copies and puts them first in `XDG_DATA_DIRS`; ft-floatd rewrites them when apps change. Tried on the live desktop: Dolphin launched floating, came back at its remembered pose after closing, and Konsole floated from `ft-float run`; the right-click entry needs a desktop start to show. The remembered scale is applied since 2026-10-01 (next item).
 - Screens hidden one at a time (2026-09-30, branch `screen-hide`): ft-screens' `conceal`/`reveal`/`concealed`, `ft-layout hide|show N`, Screens shown in Display Settings, and ft-floatd floats a new window that opens on a hidden screen (and puts strays on a screen that shows).
+- KWin's placement memory kept out (2026-10-01, branch `float-tracker`): the scale loop below turned out to be KWin's PlacementTracker, and resizing a floating window back to an earlier size set it off too. The script now puts windows back after KWin changes the outputs (see A floating window). With that, a launched app and a profile get their remembered scale back, a resize by the window's edge works (the output follows when the drag ends), and full screen on a floating window no longer flips. Tried on the live desktop with the headset off: resizing back and forth, scale steps up and down, launching at a remembered scale of 1.2, a profile putting a window's scale back, floating and docking one window while others stay put, full screen on and off, and an edge drag.
 
 Not built yet: phase 2 (the ghost, tear-off by dragging, push-flush docking), the rest of phase 3 (new windows of a floating app placed where that app's last went), and phase 4.
 
-Known problems:
-
-- **Changing a floating window's scale can loop** (Meta+scroll, or the scale buttons). Seen on the live desktop on 2026-09-30 with Dolphin at 1.2: the window flipped between 1187 and 1424 logical pixels wide, and its output between 1687 and 1925, several times a second, until it was closed. The cause looks like this: when an output resizes, KWin resizes a window on it to keep its distance from both edges. `follow` then resizes the output from a window size that's already stale. The next output change resizes the window again, and the two stay one step out of phase. To fix: have ft-floatd own the window's size while it floats. After resizing the output, treat a window change by the same amount as KWin's echo and put the window back, instead of following it. Until then, a launched app doesn't get its remembered scale back.
+Known problems: none open. The scale loop seen on 2026-09-30 (Dolphin flipping between 1187 and 1424 logical pixels wide, its output between 1687 and 1925) was KWin's placement memory, fixed on 2026-10-01.
 
 Build order from 2026-09-30, each merged into `experimental` when done: (1) the float key and docking everything, (2) the title bar button, (3) phase 3, (4) hiding screens one at a time, (5) profiles (`docs/profiles.md`).
 
@@ -115,7 +114,8 @@ Program names stay within 15 characters (`ft-floatd`). Overlay keys are `frameto
 - **Size and scale.** The panel's width is the window's pixel width times the source screen's metres per pixel, so text stays the same size in VR. A window launched floating uses the primary screen's density. Each window also has a scale (KWin's output scale), changed with Meta+scroll over the window or +/- on its bar and remembered for each app. A bigger scale makes the content bigger at the same panel size.
 - **Window state.** An ordinary window, not maximized, placed inside its output with the margin around it, and set to show on all virtual desktops. It keeps its title bar and border. Apps that draw their own title bar (GTK, Chromium) keep theirs.
 - **Moving.** Press the title bar. KWin starts an interactive move and the script reports it. ft-screens then stops forwarding pointer motion to KWin, so KWin's pointer stays at the press point and the window moves by nothing. Meanwhile ft-screens carries the panel with the pressing device, the same way the bar does today: it follows rigidly, scroll pushes and pulls, and the 3D mouse's right-drag tilts. When the button comes up, KWin gets the release at the press point. The bar under the panel works too.
-- **Resizing.** The window's own edges (inside the margin, so KWin's resize works as on the desktop) and Frametop's corner tab both change the window's size in pixels at the same density, so the app lays itself out again. ft-floatd resizes the output to keep the margin, and the panel grows or shrinks around the window's top-left corner. A screen's tab only scales the panel. Resizing is throttled to about 20 updates a second, with a minimum of 320 × 200, like screens.
+- **Resizing.** The window's own edges (inside the margin, so KWin's resize works as on the desktop) and Frametop's corner tab both change the window's size in pixels at the same density, so the app lays itself out again. ft-floatd resizes the output to keep the margin, and the panel grows or shrinks around the window's top-left corner. KWin ends a resize by the window's edge whenever an output changes, so during one the panel follows the window and the output only when the drag ends: the margin is the room to grow until then. A screen's tab only scales the panel. Resizing is throttled to about 20 updates a second, with a minimum of 320 × 200, like screens.
+- **KWin's placement memory.** KWin keeps each window's geometry, full screen and maximized state for each layout of the outputs (every enabled output's name and geometry: its `PlacementTracker`), and when the outputs come back to a layout it has seen, it puts every window back as it was in it. That's for plugging monitors in and out. Here a spare output changes size after its window does, so what KWin keeps for a spare's size is the window's next size, and going back to an earlier size made the window and its output flip between two sizes for good. Full screen did the same, and floating or docking one window could move others, even onto a spare or off one. So the KWin script keeps where each window belongs (where ft-floatd put it, or where it went outside an output change), reports nothing while KWin changes the outputs, and once KWin is done (`screensChanged` comes after its restore) puts floating windows back, and the screens' windows too when only spares changed. KWin's resize request hasn't reached the app by then, so the app never sees it. A size asked for is held for a second, since an app can still answer an older request, and then the script takes the size the window has.
 - **Full screen.** The window fills its own panel: the margin drops to zero while it's full screen, and the output is the panel's size in pixels. The panel keeps its size and place. On leaving full screen, the margin comes back.
 - **Buttons.** The close button closes the window. "Back to desktop" docks it where it came from.
 - **Minimize.** Minimizing, from the title bar or the taskbar, hides the panel, and restoring it shows the panel again. Floating windows stay in the desktop's taskbar and in Alt+Tab.
@@ -229,6 +229,6 @@ The drag proxy, the glow toward a window activated out of view (and the setting
 
 ## Risks
 
-- A SteamOS update can change KWin's script API or its nested backend. The script and the output handling are the parts to recheck after one.
+- A SteamOS update can change KWin's script API or its nested backend. The script and the output handling are the parts to recheck after one, and KWin's placement memory with them (the script undoes it, see A floating window).
 - GPU memory: each floating output has its own swapchain of two or three buffers, including the margin. The Frame has 16 GB shared, with about 4 GB free in normal use (2026-09-29).
 - Frame pacing with many panels hasn't been measured (already an open question in design.md). Each output is a separate render pass in KWin.